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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> Yeah, I think keeping at least the modules build would be important, as they are quite under-tested in the clang test suite, and the libc++ modules build often catches issues in that area, including serialization.

What about only running these configurations after changes to clang?

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