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

host_vars/runner1.archlinux.org/vault_gitlab_runner.yml

 $ANSIBLE_VAULT;1.1;AES256
-33613230393739666464663864633664666338396131346666303534373135613631356135393866
-6165346464343566343032313164353633356438623138620a306162643638633531386535333130
-65373661383734663531323234323263663138303731383464333132623730383638343064366234
-6264633765363861390a623434386134316264366665623130643536313038383463383032366638
-34306266373938663633626461373766383035376236653363346163326435396361313836666631
-31636139363737633336643664316538366133313262393434393234386636626434393430376636
-653137373335333937386137323535353938
+61636431316239363231303761363262633837373932326334393633333736643963383337376130
+6335363631313932613366313031363432636131333266660a633033656333366161323233303138
+33646530353965323836333938396633303165636661636235616266363830313433336539303361
+3761303238313331610a306338653339613431646138383063626234336632353932343539363631
+66646436383439633533393739366134623734623137333862353932343838656334613966373231
+38653531633334663662633438346363396663346438343132366637366334383564303361633436
+32316462323364393331623839633230653466623733346164633234386363616132303237353033
+66373637386365393961643932613438633036383137663937643638636566363435333563386132
+63373938333261316430623933656239383361663731373066393037613963333465