treewide: Add passthru.updateScript to gnome pkgs

`updateScript` lets us update packages automatically.
For packages that are hosted on the gnome mirror it's very easy to add
that. Inspired by https://github.com/NixOS/nixpkgs/issues/36150

pkgs/applications/misc/pdfmod/default.nix

@@ -4,11 +4,11 @@
 }:
 
 stdenv.mkDerivation rec {
-  name = "pdfmod-${version}";
+  pname = "pdfmod";
   version = "0.9.1";
 
   src = fetchurl {
-    url = "mirror://gnome/sources/pdfmod/0.9/pdfmod-${version}.tar.bz2";
+    url = "mirror://gnome/sources/${pname}/${stdenv.lib.versions.majorMinor version}/${pname}-${version}.tar.bz2";
     sha256 = "eb7c987514a053106ddf03f26544766c751c801d87762909b36415d46bc425c9";
   };