desktop: Add grim

Useful for screenshots in desktop environments that don't save
screenshots by default, such as Phosh.

modules/desktop.nix

@@ -174,7 +174,13 @@ in
       }))
 
       (attrValues {
-        inherit (pkgs) anki kanjidraw pulseaudio glib;
+        inherit (pkgs)
+          anki
+          kanjidraw
+          pulseaudio
+          glib
+          grim
+          ;
         inherit (pkgs.xfce) exo;
       })
     ];