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

.gitlab/issue_templates/New Official Project.md

@@ -93,4 +93,5 @@ If you want to add a new official project, here are some guidelines to follow:
 1. [ ] In the GitHub description of the mirrored project, append " (read-only mirror)" so that people know it's a mirror.
 1. [ ] Disable `Packages` and `Environments` from being shown on the main page.
 1. [ ] In the website field put the full url to the repository on our GitLab.
-½. [ ] Go to https://github.com/archlinux/my-example/settings/access and remove the GitHub account `archlinux-github`
+1. [ ] Go to https://github.com/archlinux/my-example/settings/access and remove the GitHub account `archlinux-github`
+1. [ ] Go to https://github.com/orgs/archlinux/teams/read-only-mirrors/repositories and add the repository with `write` permission