git-igitt: add update script

pkgs/by-name/gi/git-igitt/package.nix

@@ -6,6 +6,7 @@
   libgit2,
   oniguruma,
   zlib,
+  nix-update-script,
 }:
 
 let
@@ -36,6 +37,8 @@ rustPlatform.buildRustPackage {
     RUSTONIG_SYSTEM_LIBONIG = true;
   };
 
+  passthru.updateScript = nix-update-script { };
+
   meta = {
     description = "Interactive, cross-platform Git terminal application with clear git graphs arranged for your branching model";
     homepage = "https://github.com/mlange-42/git-igitt";