Adding default.nix for flake-compat

[arthsmn]

This way non-flake users can access packages with niv or npins easily, and as flake-compat is already a dependency, this is just a "glue". Copy-pasted from here.

An example with npins would be:
npins add github ghostty-org ghostty

and then in the config:

{pkgs, ...}: let
    sources = import ./npins;
    ghostty = import sources.ghostty;
in {
    environment.systemPackages = [ ghostty.packages.${pkgs.system}.default ];
}