ci: only use metal runners for QEMU E2E tests

We've split the QEMU E2E tests into a separate job, which we only run on merge trains and on main. This means the MR pipelines run much faster (because there's less parallelism) and can run on the dind runners (which are cheaper than the metal ones).

We've also reduced the number of jobs we run on an average pipeline from 8 to 4, courtesy of https://gitlab.arm.com/ci/component/bazelisk/-/merge_requests/21.

I've decided to use paths to filter the QEMU tests instead of tags, because it's lower maintenance. However, I couldn't figure out a nice way to not filter out //qemu for tests running on the root folder (ROOT=.) of the repository. Something for later.