[libcxx] Install runner last when building CI containers
This patch changes when we install the GHA runner in the CI containers. Instead
of having it in the base image, we install it last. This will enable a follow up
patch that will do some setup enabling building the full container image with an
existing base image, thus enabling updating the GHA runner without modifying the
important bits.
Reviewers: EricWF, ldionne
Reviewed By: ldionne
Pull Request: https://github.com/llvm/llvm-project/pull/148072
