topiary: Add passthru.updateScript

Allows to use standard

```
  nix-shell maintainers/scripts/update.nix --argstr package topiary
```

to update topiary

pkgs/development/tools/misc/topiary/default.nix

@@ -1,6 +1,7 @@
 { lib
 , rustPlatform
 , fetchFromGitHub
+, nix-update-script
 }:
 
 rustPlatform.buildRustPackage rec {
@@ -42,6 +43,8 @@ rustPlatform.buildRustPackage rec {
     install -Dm444 languages/* -t $out/share/languages
   '';
 
+  passthru.updateScript = nix-update-script { };
+
   meta = with lib; {
     description = "A uniform formatter for simple languages, as part of the Tree-sitter ecosystem";
     homepage = "https://github.com/tweag/topiary";