`] build PyCall` does not work on NixOS

I'm running into some unexpected hiccups building PyCall.jl on NixOS. I have successfully used PyCall.jl on NixOS in the past, so I'm a bit stumped as to why it's borked now. I get the following failure:
```
(@v1.7) pkg> build PyCall
Building Conda ─→ `~/.julia/scratchspaces/44cfe95a-1eb2-52ea-b672-e2afdf69b78f/6cdc8832ba11c7695f494c9d9a1c31e90959ce0f/build.log`
Building PyCall → `~/.julia/scratchspaces/44cfe95a-1eb2-52ea-b672-e2afdf69b78f/71fd4022ecd0c6d20180e23ff1b3e05a143959c2/build.log`
ERROR: Error building `PyCall`:
PREFIX=/home/skainswo/.julia/conda/3
Unpacking payload ...
cannot execute /home/skainswo/.julia/conda/3/conda.exe: NIX_LD or NIX_LD_x86_64-linux is not set
cannot execute /home/skainswo/.julia/conda/3/conda.exe: NIX_LD or NIX_LD_x86_64-linux is not set
┌ Info: No system-wide Python was found; got the following error:
│ Base.IOError("could not spawn setenv(`/nix/store/nki9ywqzbvz68vr75kn2r7g1q84f5agy-python3-3.9.6/bin/python3 -c \"import distutils.sysconfig; print(distutils.sysconfig.get_config_var('VERSION'))\"`,[\"CXX=g++\", \"PATH=/nix/store/79zvzq15mga6bgk5phnmw0fd2pnpim9v-bash-interactive-5.1-p8/bin:/nix/store/f1n2zyq47sjn0qfhxvrdawyzlgk4sgax-patchelf-0.13/bin:/nix/store/7wn7xx8faqirzkvh5iwj26b024jzg09p-gcc-wrapper-10.3.0/bin:/nix/store/v819nrv8d33ns36gm4v9vqydq1v95axi-gcc-10.3.0/bin:/nix/store/dhj23pp1sld19shjslkzw3y9pr3qpi61-glibc-2.33-59-bin/bin:/nix/store/abc8hz0v47jzsf048j1qx7m94bxfhpl7-coreutils-9.0/bin:/nix/store/gxd0gvw88wyl9bqjc03ypm5zs7qbaf74-binutils-wrapper-2.35.2/bin:/nix/store/8d30avp4xj1w8krq7zax1mlxgs28663y-binutils-2.35.2/bin:/nix/store/qmlbjq46nhn1v08nnb23y2myrvra8hm7-julia-bin-1.7.1/bin:/nix/store/rppr9s436950i1dlzknbmz40m2xqqnxc-python3-3.9.9/bin:/nix/store/abc8hz0v47jzsf048j1qx7m94bxfhpl7-coreutils-9.0/bin:/nix/store/r87rxnys39173q5nrlqq46vpa2203rmk-findutils-4.8.0/bin:/nix/store/xgqrjhz9sx8bllvayz18sql2dfd89q4g-diffutils-3.8/bin:/nix/store/aw3vgxn4fdrjqgmc94fr582hjhr2z3p3-gnused-4.8/bin:/nix/store/rvdb7057bmg3rasbqq8lzl10dffy3p31-gnugrep-3.7/bin:/nix/store/3qmgmnz3msrk0cf0qgcyja0wzyfhpxc2-gawk-5.1.1/bin:/nix/store/xc1dbg8mc8hd6bq7zizxf5nyrljk8az0-gnutar-1.34/bin:/nix/store/1wads5y31dky89hxnn5zmiri3c72kdpc-gzip-1.11/bin:/nix/store/l8d7zz3ninmgg71738q485anfjh5yd6q-bzip2-1.0.6.0.2-bin/bin:/nix/store/ba439awj33pw4l4bj95k4dy58nx69b8v-gnumake-4.3/bin:/nix/store/iqprjr5k5385bhf1dzj07zwd5p43py1n-bash-5.1-p12/bin:/nix/store/22avws4xw0chqniqym3q0w4nhdcr0hh2-patch-2.7.6/bin:/nix/store/aq0v28n1ka8m1ayg6wrpbjy00bb7fj3k-xz-5.2.5-bin/bin\", \"HOST_PATH=/nix/store/qmlbjq46nhn1v08nnb23y2myrvra8hm7-julia-bin-1.7.1/bin:/nix/store/rppr9s436950i1dlzknbmz40m2xqqnxc-python3-3.9.9/bin:/nix/store/abc8hz0v47jzsf048j1qx7m94bxfhpl7-coreutils-9.0/bin:/nix/store/r87rxnys39173q5nrlqq46vpa2203rmk-findutils-4.8.0/bin:/nix/store/xgqrjhz9sx8bllvayz18sql2dfd89q4g-diffutils-3.8/bin:/nix/store/aw3vgxn4fdrjqgmc94fr582hjhr2z3p3-gnused-4.8/bin:/nix/store/rvdb7057bmg3rasbqq8lzl10dffy3p31-gnugrep-3.7/bin:/nix/store/3qmgmnz3msrk0cf0qgcyja0wzyfhpxc2-gawk-5.1.1/bin:/nix/store/xc1dbg8mc8hd6bq7zizxf5nyrljk8az0-gnutar-1.34/bin:/nix/store/1wads5y31dky89hxnn5zmiri3c72kdpc-gzip-1.11/bin:/nix/store/l8d7zz3ninmgg71738q485anfjh5yd6q-bzip2-1.0.6.0.2-bin/bin:/nix/store/ba439awj33pw4l4bj95k4dy58nx69b8v-gnumake-4.3/bin:/nix/store/iqprjr5k5385bhf1dzj07zwd5p43py1n-bash-5.1-p12/bin:/nix/store/22avws4xw0chqniqym3q0w4nhdcr0hh2-patch-2.7.6/bin:/nix/store/aq0v28n1ka8m1ayg6wrpbjy00bb7fj3k-xz-5.2.5-bin/bin\", \"doInstallCheck=\", \"CC=gcc\", \"_PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata__linux_x86_64-linux-gnu\", \"outputs=out\", \"TMP=/run/user/1000\", \"OBJCOPY=objcopy\", \"nobuildPhase=echo\\necho \\\"This derivation is not meant to be built, aborting\\\";\\necho\\nexit 1\\n\", \"SSL_CERT_FILE=/nix/store/ggqkxkqjn8hf7h1kgv5qz8lwas9i5z6k-nss-cacert-3.71/etc/ssl/certs/ca-bundle.crt\", \"USER=skainswo\", \"NIX_CC_WRAPPER_TARGET_HOST_x86_64_unknown_linux_gnu=1\", \"SYSTEM_CERTIFICATE_PATH=/nix/store/ggqkxkqjn8hf7h1kgv5qz8lwas9i5z6k-nss-cacert-3.71/etc/ssl/certs/ca-bundle.crt\", \"depsBuildTargetPropagated=\", \"LD=ld\", \"PYTHONHASHSEED=0\", \"NIX_BINTOOLS_WRAPPER_TARGET_HOST_x86_64_unknown_linux_gnu=1\", \"IN_NIX_SHELL=pure\", \"NM=nm\", \"STRIP=strip\", \"OBJDUMP=objdump\", \"XDG_DATA_DIRS=/nix/store/f1n2zyq47sjn0qfhxvrdawyzlgk4sgax-patchelf-0.13/share\", \"nativeBuildInputs=\", \"SHELL=/nix/store/79zvzq15mga6bgk5phnmw0fd2pnpim9v-bash-interactive-5.1-p8/bin/bash\", \"system=x86_64-linux\", \"depsBuildBuild=\", \"name=nix-shell\", \"RANLIB=ranlib\", \"TMPDIR=/run/user/1000\", \"NIX_SSL_CERT_FILE=/nix/store/ggqkxkqjn8hf7h1kgv5qz8lwas9i5z6k-nss-cacert-3.71/etc/ssl/certs/ca-bundle.crt\", \"buildInputs=/nix/store/ggqkxkqjn8hf7h1kgv5qz8lwas9i5z6k-nss-cacert-3.71 /nix/store/qmlbjq46nhn1v08nnb23y2myrvra8hm7-julia-bin-1.7.1 /nix/store/rppr9s436950i1dlzknbmz40m2xqqnxc-python3-3.9.9\", \"NIX_BUILD_TOP=/run/user/1000\", \"depsHostHostPropagated=\", \"depsTargetTargetPropagated=\", \"doCheck=\", \"depsBuildTarget=\", \"SOURCE_DATE_EPOCH=315532800\", \"STRINGS=strings\", \"HOME=/home/skainswo\", \"TERM=xterm-256color\", \"TEMP=/run/user/1000\", \"PYTHONIOENCODING=UTF-8\", \"PYTHONNOUSERSITE=1\", \"_PYTHON_HOST_PLATFORM=linux-x86_64\", \"NIX_CFLAGS_COMPILE= -frandom-seed=7q7jlhbyfg -isystem /nix/store/qmlbjq46nhn1v08nnb23y2myrvra8hm7-julia-bin-1.7.1/include -isystem /nix/store/rppr9s436950i1dlzknbmz40m2xqqnxc-python3-3.9.9/include -isystem /nix/store/qmlbjq46nhn1v08nnb23y2myrvra8hm7-julia-bin-1.7.1/include -isystem /nix/store/rppr9s436950i1dlzknbmz40m2xqqnxc-python3-3.9.9/include\", \"DETERMINISTIC_BUILD=1\", \"patches=\", \"READELF=readelf\", \"AS=as\", \"__ETC_PROFILE_SOURCED=1\", \"depsHostHost=\", \"TEMPDIR=/run/user/1000\", \"NIX_INDENT_MAKE=1\", \"strictDeps=\", \"shell=/nix/store/iqprjr5k5385bhf1dzj07zwd5p43py1n-bash-5.1-p12/bin/bash\", \"NIX_BINTOOLS=/nix/store/gxd0gvw88wyl9bqjc03ypm5zs7qbaf74-binutils-wrapper-2.35.2\", \"SIZE=size\", \"LOGNAME=skainswo\", \"SHLVL=2\", \"NIX_CC=/nix/store/7wn7xx8faqirzkvh5iwj26b024jzg09p-gcc-wrapper-10.3.0\", \"_=/nix/store/qmlbjq46nhn1v08nnb23y2myrvra8hm7-julia-bin-1.7.1/bin/julia\", \"PAGER=less\", \"configureFlags=\", \"depsTargetTarget=\", \"AR=ar\", \"NIX_ENFORCE_NO_NATIVE=1\", \"PWD=/home/skainswo/dev/research/julia/permuted_sgd\", \"NIX_STORE=/nix/store\", \"NIX_BUILD_CORES=2\", \"PYTHONPATH=/nix/store/rppr9s436950i1dlzknbmz40m2xqqnxc-python3-3.9.9/lib/python3.9/site-packages\", \"out=/nix/store/7q7jlhbyfgim8i3w055lihzwxqlidxh6-nix-shell\", \"builder=/nix/store/iqprjr5k5385bhf1dzj07zwd5p43py1n-bash-5.1-p12/bin/bash\", \"propagatedBuildInputs=\", \"depsBuildBuildPropagated=\", \"shellHook=\", \"phases=nobuildPhase\", \"CONFIG_SHELL=/nix/store/iqprjr5k5385bhf1dzj07zwd5p43py1n-bash-5.1-p12/bin/bash\", \"NIX_LDFLAGS=-rpath /nix/store/7q7jlhbyfgim8i3w055lihzwxqlidxh6-nix-shell/lib64 -rpath /nix/store/7q7jlhbyfgim8i3w055lihzwxqlidxh6-nix-shell/lib -L/nix/store/qmlbjq46nhn1v08nnb23y2myrvra8hm7-julia-bin-1.7.1/lib -L/nix/store/rppr9s436950i1dlzknbmz40m2xqqnxc-python3-3.9.9/lib -L/nix/store/qmlbjq46nhn1v08nnb23y2myrvra8hm7-julia-bin-1.7.1/lib -L/nix/store/rppr9s436950i1dlzknbmz40m2xqqnxc-python3-3.9.9/lib\", \"NIX_HARDENING_ENABLE=fortify stackprotector pic strictoverflow format relro bindnow\", \"JULIA_LOAD_PATH=@:/run/user/1000/jl_h44YES\", \"propagatedNativeBuildInputs=\", \"stdenv=/nix/store/lwcfw5vyjlzrs35k1hanv21j1q2s5c5w-stdenv-linux\", \"LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:mi=00:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arc=01;31:*.arj=01;31:*.taz=01;31:*.lha=01;31:*.lz4=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.tzo=01;31:*.t7z=01;31:*.zip=01;31:*.z=01;31:*.dz=01;31:*.gz=01;31:*.lrz=01;31:*.lz=01;31:*.lzo=01;31:*.xz=01;31:*.zst=01;31:*.tzst=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.alz=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.cab=01;31:*.wim=01;31:*.swm=01;31:*.dwm=01;31:*.esd=01;31:*.jpg=01;35:*.jpeg=01;35:*.mjpg=01;35:*.mjpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.webp=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=00;36:*.au=00;36:*.flac=00;36:*.m4a=00;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=00;36:*.wav=00;36:*.oga=00;36:*.opus=00;36:*.spx=00;36:*.xspf=00;36:\", \"OPENBLAS_MAIN_FREE=1\"]): no such file or directory (ENOENT)", -2)
└ using the Python distribution in the Conda package
[ Info: Downloading miniconda installer ...
[ Info: Installing miniconda ...
ERROR: LoadError: failed process: Process(`/home/skainswo/.julia/conda/3/installer.sh -b -f -p /home/skainswo/.julia/conda/3`, ProcessExited(1)) [1]

Stacktrace:
[1] pipeline_error
@ ./process.jl:531 [inlined]
[2] run(::Cmd; wait::Bool)
@ Base ./process.jl:446
[3] run
@ ./process.jl:444 [inlined]
[4] _install_conda(env::String, force::Bool)
@ Conda ~/.julia/packages/Conda/1403Y/src/Conda.jl:207
[5] _install_conda(env::String)
@ Conda ~/.julia/packages/Conda/1403Y/src/Conda.jl:193
[6] runconda(args::Cmd, env::String)
@ Conda ~/.julia/packages/Conda/1403Y/src/Conda.jl:128
[7] add(pkg::String, env::String; channel::String)
@ Conda ~/.julia/packages/Conda/1403Y/src/Conda.jl:223
[8] add (repeats 2 times)
@ ~/.julia/packages/Conda/1403Y/src/Conda.jl:222 [inlined]
[9] top-level scope
@ ~/.julia/packages/PyCall/L0fLP/deps/build.jl:79
[10] include(fname::String)
@ Base.MainInclude ./client.jl:451
[11] top-level scope
@ none:5
in expression starting at /home/skainswo/.julia/packages/PyCall/L0fLP/deps/build.jl:43
```
It appears to be hung up on some aspect of the conda installation. However, building Conda.jl works without any issues:
```
(@v1.7) pkg> build Conda
Building Conda → `~/.julia/scratchspaces/44cfe95a-1eb2-52ea-b672-e2afdf69b78f/6cdc8832ba11c7695f494c9d9a1c31e90959ce0f/build.log`
(@v1.7) pkg>
```
A few things are weird to me about this situation:
* Why is PyCall.jl failing to build due to issues with Conda while building Conda.jl works just fine?
* `cannot execute /home/skainswo/.julia/conda/3/conda.exe: NIX_LD or NIX_LD_x86_64-linux is not set` suggests to me that Conda.jl is smart enough to use [nix-ld](https://github.com/Mic92/nix-ld) when it's available, but I haven't been able to find anything about this in the docs or in the sources for Conda.jl or PyCall.jl. Where is this warning coming from?
* The error suggests that it's trying and failing to run `/nix/store/nki9ywqzbvz68vr75kn2r7g1q84f5agy-python3-3.9.6/bin/python3 -c \"import distutils.sysconfig; print(distutils.sysconfig.get_config_var('VERSION'))\"`, but this is very odd since I don't even have that version of python installed! I have python 3.9.9 with a different hash entirely! How is it hallucinating this python install that does not exist?

The main thing that has changed since the last time this worked for me is that I upgraded NixOS from 21.05 to 21.11.

Here's my shell.nix for reference:
```nix
let
# Last updated: 1/3/22. From status.nixos.org.
pkgs = import (fetchTarball ("https://github.com/NixOS/nixpkgs/archive/adf7f03d3bfceaba64788e1e846191025283b60d.tar.gz")) { };
in
pkgs.mkShell {
buildInputs = with pkgs; [
# See https://github.com/NixOS/nixpkgs/issues/66716. Necessary for julia to
# be able to download packages.
cacert

julia_17-bin

# Necessary for PyPlot.jl
python3
];
}
```
and here's the version of python that's actually available:
```
[nix-shell:~/dev/research/julia/permuted_sgd]$ which python
/nix/store/rppr9s436950i1dlzknbmz40m2xqqnxc-python3-3.9.9/bin/python
```
```
julia> versioninfo()
Julia Version 1.7.1
Commit ac5cc99908 (2021-12-22 19:35 UTC)
Platform Info:
OS: Linux (x86_64-pc-linux-gnu)
CPU: Intel(R) Xeon(R) Platinum 8259CL CPU @ 2.50GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-12.0.1 (ORCJIT, skylake-avx512)
```

Reproduce `pkg> build PyCall` in the provided NixOS `shell.nix`, then inspect `deps/build.jl` in PyCall and the referenced `Conda.jl` entry points around `_install_conda` and `runconda`. Trace the failed `conda.exe` and Python process before checking how the NixOS upgrade affects them. Done means PyCall builds successfully in this environment with the reported Conda behavior understood.

索引モデルが issue の本文から書いたものです。