The service was enabled in arch-boxes to account for "hardware clock is
not in UTC, but instead UTC+X"[1], in our case the (VM) hardware clock
is in UTC and we therfor don't need the slow systemd-time-wait-sync
service (+30 seconds).

[1] arch-boxes@e23d3c57

Enable kernel lockdown in confidentiality mode to restrict how the root user can interact with the kernel.
See https://wiki.archlinux.org/title/Security#Kernel_lockdown_mode and https://man.archlinux.org/man/kernel_lockdown.7

This could prevent a scenario where a malicious kernel module or access to some interface that kernel lockdown prevents, would allow or assist in escaping the KVM.
It is not very likely as there needs to be an exploitable vulnerability in the hypervisor.
To make it more secure, the host too would need to enable kernel lockdown.

In the end this may only give some sense of security, but, as we all know, that's all that matters anyway.

"Disabling revoked keys in keyring" when running "pacman-key --populate"
is very slow (easily +20 seconds), in our case the boot is now ~27
seconds faster (tested on secure-runner1). The pacman master private key
is removed to prevent malicious actors from injecting packages, a new
key is generated by pacman-init.service on boot.

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