[llvm] [CI] Switch to a single runtimes build (PR #131913)

[Matheus Izvekov via llvm-commits]

mizvekov wrote:

I'd love if we could keep a way to pre-commit test these other configurations optionally, off-by-default, by for example requiring the user to add certain tags to the PR.

https://github.com/llvm/llvm-project/pull/131913