Fixes: 7ea1eb29 ("gitlab_runner: Refactor libvirt-executor")

Changes:
- Switch to arch-boxes' base image
- Verify the base image's signature
- Use the new "latest" symlink, instead of parsing the HTML for
  finding the latest arch-boxes image[1]
- Create the base image by using arch-chroot and friends, instead of
  creating a full-blown VM
- Create the VMs from domain XML template instead of virt-clone
- Switch mirror to geo.mirror.pkgbuild.com
- Try to follow "filesystem hierarchy" standards for where to place
  configuration (id_ed25519) and "vendor data" (arch-boxes.asc and
  domain_template.xml)
- Use a ed25519 key instead of RSA key
- Only start the "update base image" server if network and DNS are up
- Misc fixes and cleanups

[1] !552

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

Add an hour of delay to spread out borg backup jobs from multiple
servers and other jobs which are scheduled to run at OnCalendar=daily.