[llvm] [GitHub][docker] Add python3 venv package to CI container (PR #161024)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

LGTM.

I think we want to eventually move to separate containers for the clang-format/premerge/clang-tidy workflows to trim down on space (and preinstall necessary python dependencies), but this seems fine for now, especially since the premerge container already has clang-format/clang-tidy.

https://github.com/llvm/llvm-project/pull/161024