clipper2: add passthru.updateScript

Hopefully helps prevent r-ryantm from making mistakes like in 413003.

pkgs/by-name/cl/clipper2/package.nix

@@ -3,6 +3,7 @@
   stdenv,
   fetchFromGitHub,
   cmake,
+  nix-update-script,
 }:
 
 stdenv.mkDerivation rec {
@@ -28,6 +29,8 @@ stdenv.mkDerivation rec {
     "-DBUILD_SHARED_LIBS=ON"
   ];
 
+  passthru.updateScript = nix-update-script { };
+
   meta = {
     description = "Polygon Clipping and Offsetting - C++ Only";
     longDescription = ''