fheroes2: add trivial updater script

pkgs/games/fheroes2/default.nix

@@ -1,5 +1,8 @@
 { stdenv, lib, fetchFromGitHub
 , gettext, libpng, SDL2, SDL2_image, SDL2_mixer, SDL2_ttf, zlib
+
+# updater only
+, nix-update-script
 }:
 
 stdenv.mkDerivation rec {
@@ -29,6 +32,12 @@ stdenv.mkDerivation rec {
     runHook postInstall
   '';
 
+  passthru = {
+    updateScript = nix-update-script {
+      attrPath = pname;
+    };
+  };
+
   meta = with lib; {
     homepage = "https://github.com/ihhub/fheroes2";
     description = "Free implementation of Heroes of Might and Magic II game engine";