infra: Get NixOS VM tests to run again

[fricklerhandwerk]

GitHub can't run NixOS tests because that requires KVM. We do have a machine at Hetzner though. Currently Hydra is running on that, but essentially all builds fail for unclear reasons.

• Set up https://github.com/Mic92/buildbot-nix to implement fine-grained CI
  • This is what ci: Use Build Matrix #193 is trying to address.
• Check if build jobs are still needed, remove those for archived flake repos from past years
• Shut down our Hydra instance https://hydra.ngi0.nixos.org
• Place infra code into the monorepo, guard merge access through CODEOWNERS
  • existing code, largely outdated, but needs to be checked:
    • https://github.com/ngi-nix/ngi0-infra
    • https://github.com/ngi-nix/infra
• Add documentation for how to get access to the relevant resources, who to ask, etc.
  • We should make sure that our usage of sops-nix is documented
  • We also need to document how changes to makemake are applied