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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Looks like things scale reasonably well when running in CI. I haven't done exact A/B testing with the container vs without it yet, so it's nice to see the gains are pretty significant.

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