tweak(emacs): use emacs from nixpkgs

So we don't have to rebuild a rolling version of Emacs which is
incredibly unlikely to be cached due to the override.

modules/nixos/programs/emacs/default.nix

@@ -11,7 +11,7 @@ in
   config =
     let
       emacsPackage = pkgs.emacsWithPackagesFromUsePackage {
-        package = pkgs.emacs-pgtk.override { withImageMagick = true; };
+        package = pkgs.emacs29-pgtk.override { withImageMagick = true; };
         config = ./init.el;
         alwaysEnsure = true;
         override = pkgs.callPackage ./overlay.nix { };