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

-#!/usr/bin/evn bash
+#!/usr/bin/env bash
 set -o nounset -o errexit -o pipefail
 readonly MIRROR="https://mirror.pkgbuild.com"
 readonly LIBVIRT_DEFAULT_POOL_PATH="/var/lib/libvirt/images"
@@ -22,10 +22,6 @@ get_vm_name() {
 
 wait_for_ssh() {
   for i in {1..90}; do
-    if [ "${i}" -eq 90 ]; then
-      echo 'Waited 90 seconds for VM to start, exiting...'
-      exit "${SYSTEM_FAILURE_EXIT_CODE:-1}"
-    fi
     if ! get_vm_ip "${1}"; then
       echo "Waiting for network"
       sleep 1
@@ -36,20 +32,20 @@ wait_for_ssh() {
       sleep 1
       continue
     fi
-    break
+    return 0
   done
+  echo 'Waited 90 seconds for VM to start, exiting...'
+  exit "${SYSTEM_FAILURE_EXIT_CODE:-1}"
 }
 
 wait_for_vm_shutdown() {
     for i in {1..10}; do
-        if [ "${i}" -eq 60 ]; then
-            return 1
-        fi
         if LC_ALL=C virsh domstate "${1}" | grep -F "shut off"; then
-            break
+            return 0
         fi
         sleep 1
     done
+    return 1
 }
 
 # Create a updated VM image with the required tools