modules: Export `pkgs` to match NixOS (#2696)

This makes it a lot easier to access the `pkgs` that would be used to
build the home configuration, e.g.

    nix build ./dotfiles#homeConfigurations."user@host".pkgs.vim

This is useful as it allows access to a Nixpkgs that has been
instiantiated with config and overlays.

modules/default.nix

@@ -61,4 +61,6 @@ in
     sort (a: b: a.time > b.time) (
       filter (a: a.condition) rawModule.config.news.entries
     );
+
+  inherit (module._module.args) pkgs;
 }