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

@@ -89,7 +89,7 @@ run() {
 cleanup() {
   rm /run/libvirt-executor/id_ed25519_$(vm_name){,.pub}
   virsh destroy "$(vm_name)" || true
-  virsh undefine --remove-all-storage "$(vm_name)"
+  virsh undefine --nvram --remove-all-storage "$(vm_name)"
 }
 
 case "${1:-}" in