[llvm] [workflows] Stop using the build-test-llvm-project action (PR #80580)

[Aiden Grossman via llvm-commits]

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

LGTM. This is definitely a nice simplification. I'm assuming there will be another commit/PR in https://github.com/llvm/actions to delete this action since it won't be needed any longer?

https://github.com/llvm/llvm-project/pull/80580