birdfont: add update script

pkgs/tools/misc/birdfont/default.nix

@@ -18,6 +18,7 @@
   gsettings-desktop-schemas,
   wrapGAppsHook3,
   autoPatchelfHook,
+  nix-update-script,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
@@ -63,6 +64,8 @@ stdenv.mkDerivation (finalAttrs: {
 
   installPhase = "./install.py";
 
+  passthru.updateScript = nix-update-script { };
+
   meta = with lib; {
     description = "Font editor which can generate fonts in TTF, EOT, SVG and BIRDFONT format";
     homepage = "https://birdfont.org";