Add hydra jobs & flake packages

[RossComputerGuy]

Description of changes

Progress towards #854

This adds packages and hydraJobs to the Flake. This is a first step towards caching things on Hydra.

Things done

• [x] Tested the changes in your own NixOS Configuration
• [x] Tested the changes end-to-end by using your fork of nixos-hardware and importing it via <nixos-hardware> or Flake input