[llvm-branch-commits] [llvm] [libc++] Target the release runners set from the LLVM 23 release branch (PR #210379)

[Louis Dionne via llvm-branch-commits]

ldionne wrote:

Note that this is targeting the LLVM 23 release branch directly, NOT `main`. We want to pin the runners used by the release branch to keep them stable.

Also, this depends on the changes in https://github.com/llvm/llvm-project/pull/210378 being deployed via Terraform so that `llvm-premerge-libcxx-release-runners` actually has the desired image.

https://github.com/llvm/llvm-project/pull/210379