[llvm] Fix: [lit] builtin cat/diff spawn-fallback leaked PYTHONPATH into later RUN lines (PR #215151)

[via llvm-commits]

M001N wrote:

Thanks for the review!

Acknowledging the AI Tool Use Policy: yes, I've read
https://llvm.org/docs/AIToolPolicy.html. AI assistance is disclosed in the
PR description (the `[!INFO]` block), the change was independently
reviewed and manually verified by a human before submission, and I'm able
to answer questions about the fix and the reasoning behind it.

On the test: agreed, and this was actually a known limitation I called
out in the PR description -- the FileCheck binary isn't available in the
environment I prepared this in (it normally ships with a full LLVM/Clang
build), so I couldn't validate a FileCheck-based `.test` file locally and
went with a pure-Python `unittest` addition to `tests/unit/TestRunner.py`
instead, specifically so it stayed verifiable without that dependency.

Happy to add a proper lit `.test` file (e.g. extending `shtest-cat.py`)
covering this with `not --crash cat ...` or `env FOO=bar cat ...` as well,
once I can validate it against a real FileCheck. Would you prefer that
alongside the existing unit test, or as a replacement for it?


https://github.com/llvm/llvm-project/pull/215151