[llvm] Modify lit config to allow specify extra env vars to pass through to lit invocation (PR #208239)

[David Peixotto via llvm-commits]

https://github.com/dmpots approved this pull request.

LGTM. This looks like a useful generic addition to be able to pass env vars. It keeps everything explicit which preserves traceability.

https://github.com/llvm/llvm-project/pull/208239