Use VM runners[1] for building

Building inside a TCG accelerated qemu VM is slow and painful, but it is
the only option when running in a non-privileged container.

arch-boxes has been built inside a KVM accelerated VMs ("VM runner") for
over 11 months[2] and recently the MR[1] was merged into the
infrastructure repo. With it now being a official part of arch's
infrastructure we should switch to it and get much faster builds.

Doing some quick testing, the whole pipeline is now roughly ~29-84
minutes faster (taking between 7-9 minutes, instead of 36-93 minutes).

[1] infrastructure!385
[2] arch-boxes@3bda5b26

Fix #161