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

[Tom Stellard via llvm-commits]

tstellar wrote:

> 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?

Yeah, I can remove it from there too.

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