[llvm] Use container on Linux to run llvm-project-tests workflow (PR #81349)

[Aiden Grossman via llvm-commits]

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

One inline question, otherwise LGTM.

Do you have figures on how much this sped up the pipeline? Also, I'm assuming the plan is to enable this later for the other projects?

https://github.com/llvm/llvm-project/pull/81349