[libc++][Docker] Don't actually start using a newer compiler in the CI

To ensure a smooth transition without breaking CI, we should use explicitly
versioned Clangs in the CI jobs definition instead, since that's a change
we can actually test in the CI.

So instead of bumping the compiler version from the Docker image, use
the same version as before by default, and we can bump it from the CI
job definition once all the nodes are running the new image.