Migrating to Flakes gives us a few nice benefits:

• Inputs are actually sane: no more <acm-aws> and $NIX_PATH hack.
• Better caching via GitHub Actions using the more efficient magic-nix-cache-action
• Ability to use the easier unstable nix CLI

Plan:

• Decouple Nixpkgs from Niv and into a Flake.
• Migrate default.nix and shell.nix into flake.nix
  • shell.nix becomes #.devShells.<arch>.default
  • default.nix gets combined with #, since it already contains packages and nixosConfigurations
• Migrate Terraform files to use the Flake file