Use pgtk emacs on linux

nix/modules/emacs.nix

@@ -1,6 +1,9 @@
 { config, lib, pkgs, ... }:
 with config.lib.file;
-let emacsWithPackages = (pkgs.emacsPackagesFor pkgs.emacs).emacsWithPackages;
+let emacsWithPackages = (pkgs.emacsPackagesFor
+  (if pkgs.stdenv.hostPlatform.isDarwin
+   then pkgs.emacs
+   else pkgs.emacs-pgtk)).emacsWithPackages;
     aspell = (pkgs.aspellWithDicts (d: [d.en]));
 in
 {