[llvm] [CI] Clean up runtimes builds (PR #131913)

[Vlad Serebrennikov via llvm-commits]

Endilll wrote:

As a follow-up for this patch, I'll make Clang Modules build reuse the build directory after C++26 build, because they only differ in `LIBCXX_TEST_PARAMS` (and `LIBCXXABI_TEST_PARAMS`). I just checked locally that we don't need a clean build to make this change propagate. This should save us couple more minutes per build (out of typical 30 minutes). 

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