CI: use faster gitlab.com runners

We have access to the fancy runners since GitLab.com donate premium to us. There's no downside to making the most of them while we can. We don't depend on them, but they'll make our CI go zoom!

For the next few months until we migrate to self-hosted :D

This includes native(?) ARM64 runners, which will at least be helpful to validate that building arm64 on arm64 works.