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

[Matheus Izvekov via llvm-commits]

mizvekov 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.

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