fix: Fix whl_library in bazel vendor mode

[omar-droubi]

Resolves #3079

• Added PYTHONHOME to whl_library execution environment. Without it, the python interpreter is getting confused where it's running from when bazel --vendor_dir is used
• In _get_toolchain_unix_cflags, the real path for python is used instead of the symlink.

[omar-droubi]

Hi @aignas, with regards to the tests, the bug this PR resolves occurs when there is a "bazel clean --expunge" between the "bazel vendor" and the "bazel build" commands. Is it possible to update the "integration_test_bzlmod_ubuntu_vendor" with a "bazel clean" ? I am not sure how impactful that would be on the CI performance.

integration_test_bzlmod_ubuntu_vendor:
<<: *reusable_build_test_all
name: "examples/bzlmod: bazel vendor"
working_directory: examples/bzlmod
platform: ubuntu2204
shell_commands:
- "bazel clean --expunge"
- "bazel vendor --vendor_dir=./vendor //..."
- "bazel clean --expunge"
- "bazel build --vendor_dir=./vendor //..."
- "rm -rf ./vendor"


[aignas]

I would like to avoid adding bazel clean in the CI, maybe we could just add a line that does bazel vendor if it is not present yet. If we have regressions in the future, we can look into bazel clean usage.

[aignas]

Thank you!