maintainers/scripts/update.nix: support auto-committing by passing attrPath

Instead of having the updateScript support returning JSON object,
it should be sufficient to specify attrPath in passthru.updateScript.
It is much easier to use.

The former is now considered experimental.

maintainers/scripts/update.nix

@@ -143,8 +143,10 @@ let
   packageData = package: {
     name = package.name;
     pname = lib.getName package;
+    oldVersion = lib.getVersion package;
     updateScript = map builtins.toString (lib.toList (package.updateScript.command or package.updateScript));
     supportedFeatures = package.updateScript.supportedFeatures or [];
+    attrPath = package.updateScript.attrPath or null;
   };
 
   packagesJson = pkgs.writeText "packages.json" (builtins.toJSON (map packageData packages));