libvirt-executor improvements

A bunch of improvements to the libvirt-executor, with the major one being that the image is now built in the arch-boxes projects (arch-boxes!200 (merged)), instead of on each runner.

Please see the commit messages for more details.

---

Deployed on runner1.archlinux.org and secure-runner1.archlinux.org.

roles/gitlab_runner/files/libvirt-executor-update-base-image.timer

 [Unit]
-Description=Run libvirt-executor-vm-template.service daily
+Description=Run libvirt-executor-update-base-image.service daily
 
 [Timer]
 # One hour after the "Nightly build" pipeline