nixf-diagnose: add update script

To allow running locally:

```
nix-shell maintainers/scripts/update.nix --argstr package nixf-diagnose
```

pkgs/by-name/ni/nixf-diagnose/package.nix

@@ -3,6 +3,7 @@
   rustPlatform,
   fetchFromGitHub,
   nixf,
+  nix-update-script,
 }:
 
 rustPlatform.buildRustPackage (finalAttrs: {
@@ -20,6 +21,8 @@ rustPlatform.buildRustPackage (finalAttrs: {
 
   cargoHash = "sha256-nrr2/lTWPyH7MsG2hSMJjbFCpHsKWINEP8jwSYPhocg=";
 
+  passthru.updateScript = nix-update-script { };
+
   meta = {
     description = "CLI wrapper for nixf-tidy with fancy diagnostic output";
     mainProgram = "nixf-diagnose";