This is useful for flake users as they will usually already have an
instantiated Nixpkgs e.g.
let
pkgs = import nixpkgs {
config.allowUnfree = true;
overlays = [ ... ];
}
in darwin.lib.darwinSystem {
inherit pkgs;
}
This change makes `nix-darwin` match the behaviour of NixOS and
`home-manager`.
