Switch to VM runners

Currently the CI runs docker which in turn runs a a TCG accelerated qemu VM. This is slow and painful. The whole thing is more complicated than it should be.

We can't run mkarchiso in docker since pacstrap will fail.

@klausenbusk created a real KVM accelerated VM runner (see infrastructure#283 (closed), infrastructure!385 (merged) and arch-boxes@3bda5b26). It's faster and simpler. Let's use it!

This would obsolete #137 (closed).