[libc++] Run Linux benchmarks on the premerge k8s runner set (#227547)

The Linux benchmarking machine was still using the
llvm-premerge-libcxx-runners runner set, which doesn't exist anymore.
Moving to K8s also requires specifying the Docker image to use in
machines.json.

As a drive-by, drop the manual Python installation since a suitable
version of Python should already exist in the image.

GitOrigin-RevId: 6281eb5d56b6ffd988a4d0fb7ed9a4652a7d8af5