PyCall fails to build when using `PYTHON=""`

On the latest Julia (1.8.5) with the latest release of PyCall (1.95.1), attempting to build PyCall with `PYTHON=""` (i.e. using the conda-provided python), fails:

```
julia> ENV["PYTHON"] = ""
""

(sympy-test) pkg> build PyCall
Building Conda ─→ `~/.julia/scratchspaces/44cfe95a-1eb2-52ea-b672-e2afdf69b78f/e32a90da027ca45d84678b826fffd3110bb3fc90/build.log`
Building PyCall → `~/.julia/scratchspaces/44cfe95a-1eb2-52ea-b672-e2afdf69b78f/62f417f6ad727987c755549e9cd88c46578da562/build.log`
ERROR: Error building `PyCall`:
Traceback (most recent call last):
File "/home/rdeits/.julia/conda/3/bin/conda", line 7, in
from conda.cli import main
ModuleNotFoundError: No module named 'conda'
┌ Info: Using the Python distribution in the Conda package by default.
└ To use a different Python version, set ENV["PYTHON"]="pythoncommand" and re-run Pkg.build("PyCall").
[ Info: Running `conda install -y numpy` in root environment
ERROR: LoadError: failed process: Process(setenv(`/home/rdeits/.julia/conda/3/bin/conda install -y numpy`,
...
```

For completeness:

```
julia> versioninfo()
Julia Version 1.8.5
Commit 17cfb8e65ea (2023-01-08 06:45 UTC)
Platform Info:
OS: Linux (x86_64-linux-gnu)
CPU: 8 × Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-13.0.1 (ORCJIT, skylake)
Threads: 1 on 8 virtual cores

(sympy-test) pkg> st
Status `~/Downloads/sympy-test/Project.toml`
[438e738f] PyCall v1.95.1
```

This is on Ubuntu 20.04.