Use GCP preemptible instances for runners

We can consider if GCP preemptible VMs instances (https://cloud.google.com/compute/docs/instances/preemptible) are a good candidate for our runners, and see if the drop rate of jobs vs. the cost saving is worth the switch.

The adoption on .com is discussed in https://gitlab.com/gitlab-com/infrastructure/issues/3131, here we can track changes to runner in order to be able to run there.