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

@@ -61,10 +61,14 @@ wait_for_vm_shutdown() {
 
 # Create a updated VM image with the required tools
 create_vm_template() {
-  # FIXME: Download from arch-boxes when [1] is in
-  # [1] https://gitlab.archlinux.org/archlinux/arch-boxes/-/merge_requests/172
   local vm_name
   printf -v vm_name 'libvirt_executor_vm_template_%(%s)T_tmp'
+
+  local latest_image="$(curl -fs "${MIRROR}/images/latest/" | grep -Eo 'Arch-Linux-x86_64-cloudimg-[0-9]{8}\.[0-9]+\.qcow2'| head -n 1)"
+  if [ -z "${latest_image}" ]; then
+    echo "Error: Couldn't find latest cloud image"
+    exit 1
+  fi
   local image_path="${LIBVIRT_DEFAULT_POOL_PATH}/${vm_name}.qcow2"
   trap 'rm -f -- "${image_path}"' EXIT
   curl -sSf "${MIRROR}/images/v20210815.31636/Arch-Linux-x86_64-cloudimg-20210815.31636.qcow2" --output "${image_path}"