[llvm] Reapply "[CI] Migrate to runtimes build" (#143612) (PR #144033)

[Reid Kleckner via llvm-commits]

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

Does the CI test changes to .ci? Is the red CI check related or not? I wasn't able to load the logs to check.

https://github.com/llvm/llvm-project/pull/144033