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-update-base-image

@@ -53,6 +53,5 @@ image_path="$(printf '%s/runner-base-%(%s)T.qcow2' "${libvirt_default_pool_path}
 cp Arch-Linux-x86_64-basic.qcow2 "${image_path}.tmp"
 mv "${image_path}"{.tmp,}
 
-cd "${libvirt_default_pool_path}"
 # Keep one week of base images
 compgen -G "${libvirt_default_pool_path}/runner-base-*.qcow2" | sort -n -t - -k3,3 | head -n -7 | xargs --no-run-if-empty rm -vf