gitlab_runner: Add VM based executor (libvirt-executor)

For some workloads running in a container is too restrictive, ex:
arch-boxes (loop device, filesystem mount, pacstrap) and archiso
(pacstrap). Currently they both run a TCG accelerated QEMU VM, which is
very slow and painful to work with. We should provide a better option to
our users!

This adds a hardware accelerated VM for this kinds of workloads, which
is way faster and you can do whatever you like (mostly)!

Fix #283

---

TODO:

• Enable KSM?
• Get archiso to build
  • Done: https://gitlab.archlinux.org/klausenbusk/archiso/-/pipelines/10976
• Use BUILD_FAILURE_EXIT_CODE and SYSTEM_FAILURE_EXIT_CODE: https://docs.gitlab.com/runner/executors/custom.html#error-handling
• Fix existing users to use the secure-vm tag

roles/gitlab_runner/files/libvirt-executor

@@ -58,6 +58,8 @@ wait_for_vm_shutdown() {
 
 # https://docs.gitlab.com/runner/executors/custom.html#prepare
 prepare() {
+  # shellcheck disable=SC2064
+  trap "exit ${SYSTEM_FAILURE_EXIT_CODE}" ERR
   local base_image
   base_image="$(compgen -G "${libvirt_default_pool_path}/runner-base-*.qcow2" | sort -n -t - -k3,3 | tail -n 1)"