ci: Use self-hosted runners for CI (esp arm testing)

[olix0r]

GitHub Actions runners can be hard to come by so that many workflows backup (at the time of writing we have 3 running and 6 queued workflows). Can we augment this with self-hosted runners? In order to do this, we'd need to make this setup reproducible, probably with Terraform, as we may have to abandon/reprovision/move these compute resources.