[llvm] Allow specifying libcxx builder image. (PR #110303)

[Louis Dionne via llvm-commits]

https://github.com/ldionne approved this pull request.

This LGTM if CI is green! Once this is merged, we should scale up the new-style runners and scale-down the old ones (but keep one cluster around for a few weeks just to finish running existing patches).

This is amazing!

https://github.com/llvm/llvm-project/pull/110303