Use self-hosted runners on the GCP to build long foreign arch jobs

[umarcor]

Building some of the tools for foreign architectures (which requires QEMU) is x5-x10 slower and can timeout on default GitHub Actions runners. Using Antmicro's self-hosted runner on the Google Cloup Platform project (gcr.io/hdl-containers) would allow to provide multiarch images for all the tools.

In order to make a sensible usage of GCP resources, #50 should be addressed first. That will allow to have fine-grained control over the machine where each job is to be run.