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

@@ -87,6 +87,7 @@ create_vm_template() {
     ssh pacman -Sy --noconfirm --needed archlinux-keyring
     ssh pacman -Syu --noconfirm git git-lfs gitlab-runner
     ssh "sed -E 's/^#(IgnorePkg *=)/\1 linux/' -i /etc/pacman.conf"
+    ssh rm /etc/machine-id /var/lib/dbus/machine-id
 
     virsh shutdown "${vm_name}"
     wait_for_vm_shutdown "${vm_name}"