Cabal >= 3.14 doesn't add build-tools with an absolute Path for Setup.hs test and v1-test

**Describe the bug**
cabal-install/Cabal add `build-tool-depends` to `PATH` in the environment of the running test suite since Cabal 3.14. For `v2-test`, this uses absolute paths while `v1-test` and `Setup.hs` use relative paths.

This breaks test suites that rely on this feature to find a certain executable and change directory during the course of their execution (e.g. use temporary directories). (See below for a list of affected Hackage packages.)

**To Reproduce**

1. Clone .
This is a simple executable that prints "hello world" and a [test suite](https://github.com/sternenseemann/reproducer-cabal-build-tools-test-suite/blob/canon/test/Main.hs) that uses it as build tool. It looks up its location and then runs it twice: once from the initial working directory and once from `./test`.
The latter will fail if the path to the build tool is not absolute.
3. Run `cabal v2-test --test-show-details=streaming` and observe everything working as expected:
```
[("PATH","/home/lukas/src/hs/reproducer-cabal-build-tools-test-suite/./dist-newstyle/build/x86_64-linux/ghc-9.12.3/reproducer-cabal-build-tools-test-suite-0/t/reproducer-cabal-build-tools-test-suite-test/build/build-tool:/home/lukas/src/hs/reproducer-cabal-build-tools-test-suite/dist-newstyle/build/x86_64-linux/ghc-9.12.3/reproducer-cabal-build-tools-test-suite-0/x/build-tool/build/build-tool:/nix/store/92jwzc69lwmlzr80vv3ssd50i0b19cbk-groff-1.23.0/bin:/nix/store/x12lw455sq6qy2wcya85d7rb88ybc3df-bash-interactive-5.3p9/bin:/nix/store/c2p7haf4zzkbrir9zs662r68c5dmylbq-patchelf-0.15.2/bin:/nix/store/a245z3cvf9x9sn0xlk6k8j9xhxbhda1z-gcc-wrapper-15.2.0/bin:/nix/store/mjf8jlq9grydcdvyw6hb063x5c34g5gf-gcc-15.2.0/bin:/nix/store/0bdqq2z98kg2hfn3k60if6pb5fd5p10h-glibc-2.42-47-bin/bin:/nix/store/i2vmgx46q9hd3z6rigaiman3wl3i2gc4-coreutils-9.9/bin:/nix/store/i6ppbrlpp6yki8qvka7nyv091xa8dchx-binutils-wrapper-2.44/bin:/nix/store/47mn80zqpygykqailwzw8zlag4cgl75q-binutils-2.44/bin:/nix/store/q6lnm5x1ji2m8vhsb43xf48vr6yd2ysd-ghc-9.12.3/bin:/nix/store/z2903hglhpxv4hyczli0ncdf09wmma92-ncurses-6.6/bin:/nix/store/h0x0p1ikp9svk1ri7xxacaczqawyz4sm-numactl-2.0.18/bin:/nix/store/i2vmgx46q9hd3z6rigaiman3wl3i2gc4-coreutils-9.9/bin:/nix/store/16wfacfgap3chf7mcjnd8dwi85dj4qqi-findutils-4.10.0/bin:/nix/store/3p87h6dn5i87i3iq9364imzbqgwvkg2p-diffutils-3.12/bin:/nix/store/ryz8kcrm2bxpccllfqlb7qldsfnqp5c2-gnused-4.9/bin:/nix/store/02vv0r262agf9j5n2y1gmbjvdf12zkl0-gnugrep-3.12/bin:/nix/store/2xq9rayckw8zq26k274xxlikn77jn60j-gawk-5.3.2/bin:/nix/store/qyg62bc2xnpwz0fa9prqxvvk00zj4g9q-gnutar-1.35/bin:/nix/store/84yyzmxs7mb8nhkvcfv9n1l9irpb6mnq-gzip-1.14/bin:/nix/store/90yw24gqmwph4xjp4mqhpx1y1gcrvqla-bzip2-1.0.8-bin/bin:/nix/store/vbah5c4rzy1q1hbqhginyxjhj8d4dj8j-gnumake-4.4.1/bin:/nix/store/f15k3dpilmiyv6zgpib289rnjykgr1r4-bash-5.3p9/bin:/nix/store/wwij6563c6wbg4kzgjhng7vlhf7api19-patch-2.8/bin:/nix/store/zys6d102zp171wpwcs08g632886w2qxs-xz-5.8.2-bin/bin:/nix/store/nyy0bvgjwd98x7ih8pl6pr79qjljgsf7-file-5.45/bin:/run/wrappers/bin:/home/lukas/.nix-profile/bin:/nix/profile/bin:/home/lukas/.local/state/nix/profile/bin:/etc/profiles/per-user/lukas/bin:/nix/var/nix/profiles/default/bin:/run/current-system/sw/bin")]
/home/lukas/src/hs/reproducer-cabal-build-tools-test-suite/dist-newstyle/build/x86_64-linux/ghc-9.12.3/reproducer-cabal-build-tools-test-suite-0/x/build-tool/build/build-tool/build-tool is absolute
Hello World!
Hello World!
```
4. Run `cabal v1-test --show-details=streaming` (or the equivalent Setup.hs commands) and observe:
```
[("PATH","dist/build/build-tool:/nix/store/92jwzc69lwmlzr80vv3ssd50i0b19cbk-groff-1.23.0/bin:/nix/store/x12lw455sq6qy2wcya85d7rb88ybc3df-bash-interactive-5.3p9/bin:/nix/store/c2p7haf4zzkbrir9zs662r68c5dmylbq-patchelf-0.15.2/bin:/nix/store/a245z3cvf9x9sn0xlk6k8j9xhxbhda1z-gcc-wrapper-15.2.0/bin:/nix/store/mjf8jlq9grydcdvyw6hb063x5c34g5gf-gcc-15.2.0/bin:/nix/store/0bdqq2z98kg2hfn3k60if6pb5fd5p10h-glibc-2.42-47-bin/bin:/nix/store/i2vmgx46q9hd3z6rigaiman3wl3i2gc4-coreutils-9.9/bin:/nix/store/i6ppbrlpp6yki8qvka7nyv091xa8dchx-binutils-wrapper-2.44/bin:/nix/store/47mn80zqpygykqailwzw8zlag4cgl75q-binutils-2.44/bin:/nix/store/q6lnm5x1ji2m8vhsb43xf48vr6yd2ysd-ghc-9.12.3/bin:/nix/store/z2903hglhpxv4hyczli0ncdf09wmma92-ncurses-6.6/bin:/nix/store/h0x0p1ikp9svk1ri7xxacaczqawyz4sm-numactl-2.0.18/bin:/nix/store/i2vmgx46q9hd3z6rigaiman3wl3i2gc4-coreutils-9.9/bin:/nix/store/16wfacfgap3chf7mcjnd8dwi85dj4qqi-findutils-4.10.0/bin:/nix/store/3p87h6dn5i87i3iq9364imzbqgwvkg2p-diffutils-3.12/bin:/nix/store/ryz8kcrm2bxpccllfqlb7qldsfnqp5c2-gnused-4.9/bin:/nix/store/02vv0r262agf9j5n2y1gmbjvdf12zkl0-gnugrep-3.12/bin:/nix/store/2xq9rayckw8zq26k274xxlikn77jn60j-gawk-5.3.2/bin:/nix/store/qyg62bc2xnpwz0fa9prqxvvk00zj4g9q-gnutar-1.35/bin:/nix/store/84yyzmxs7mb8nhkvcfv9n1l9irpb6mnq-gzip-1.14/bin:/nix/store/90yw24gqmwph4xjp4mqhpx1y1gcrvqla-bzip2-1.0.8-bin/bin:/nix/store/vbah5c4rzy1q1hbqhginyxjhj8d4dj8j-gnumake-4.4.1/bin:/nix/store/f15k3dpilmiyv6zgpib289rnjykgr1r4-bash-5.3p9/bin:/nix/store/wwij6563c6wbg4kzgjhng7vlhf7api19-patch-2.8/bin:/nix/store/zys6d102zp171wpwcs08g632886w2qxs-xz-5.8.2-bin/bin:/nix/store/nyy0bvgjwd98x7ih8pl6pr79qjljgsf7-file-5.45/bin:/run/wrappers/bin:/home/lukas/.nix-profile/bin:/nix/profile/bin:/home/lukas/.local/state/nix/profile/bin:/etc/profiles/per-user/lukas/bin:/nix/var/nix/profiles/default/bin:/run/current-system/sw/bin")]
dist/build/build-tool/build-tool is not absolute
reproducer-cabal-build-tools-test-suite-test: Uncaught exception ghc-internal:GHC.Internal.IO.Exception.IOException:

dist/build/build-tool/build-tool: createProcess: posix_spawnp: does not exist (No such file or directory)

While handling createProcess: posix_spawnp: does not exist (No such file or directory)

HasCallStack backtrace:
ioError, called at libraries/process/System/Process/Common.hs:240:16 in process-1.6.26.1-27cf:System.Process.Common

Hello World!
```

Note that with GHC 9.12.2/Cabal 3.14.1.0 this issue is not quite always reproducible. Before https://github.com/haskell/cabal/pull/10827, the duplicate `PATH` entries in the environment could cause the test suite to never see the modified `PATH` environment variable. This has masked the issue in Nixpkgs so far because we add the binary to `PATH` manually as well (hence [my comment](https://github.com/haskell/cabal/pull/9912#issuecomment-3960033271)).

cc @mpickering

**Expected behavior**

I see two options:

1. Don't make `build-tool-depends` available at test suite run time where the working directory may change arbitrarily.
2. Add an absolute path to `PATH` like for `v2-test`.

**System information**
- NixOS
- Cabal 3.14.1.0, 3.14.2.0, GHC 9.12.2, 9.12.3

**Additional context**

The problem is that there are at least two projects in the wild that rely on `PATH` being updated with build tools in order to be able to test the package's main executable:

- cabal-add
- fourmolu

This is unfortunate since it's not really what the feature is intended for, but there's also hardly any better option with what Cabal offers at the moment.