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

@@ -35,4 +35,4 @@ echo "${image_name}" > /usr/local/lib/libvirt-executor/backing-vol-name.tmp
 mv /usr/local/lib/libvirt-executor/backing-vol-name{.tmp,}
 
 # Keep one week of base images
-virsh vol-list "${libvirt_pool}" | awk '$1~"Arch-Linux-x86_64-libvirt-executor-[0-9]*\\.[0-9]*\\.qcow2" {print $1}' | sort -n -t - -k3,3 | head -n -7 | xargs -I{} --no-run-if-empty virsh vol-delete {} "${libvirt_pool}"
+virsh vol-list "${libvirt_pool}" | awk '$1~"Arch-Linux-x86_64-libvirt-executor-[0-9]*\\.[0-9]*\\.qcow2" {print $1}' | sort -n -t - -k6,6 | head -n -7 | xargs -I{} --no-run-if-empty virsh vol-delete {} "${libvirt_pool}"