Overlay and NixOS module for Urbit

One way or another you need to fetch this repo from your nix sources, and then import it and add the overlay attribute to your package set. The usual way is to add it to the overlays list passed to nixpkgs.

When we have a NixOS module this will be done automatically.

Use niv to add nixos-urbit as a source:

$ niv add nixos-urbit

Then in the configuration for your package set: `{ overlays = [ ... (import sources.nixos-urbit).overlay]; } (where ... is your other overlays).