nix flake prefetch: Remove __final

This is currently an internal attribute, not intended to be shown to
users.

Fixes #13150.

src/nix/flake.cc

@@ -1497,6 +1497,7 @@ struct CmdFlakePrefetch : FlakeCommand, MixJSON
             res["hash"] = hash.to_string(HashFormat::SRI, true);
             res["original"] = fetchers::attrsToJSON(resolvedRef.toAttrs());
             res["locked"] = fetchers::attrsToJSON(lockedRef.toAttrs());
+            res["locked"].erase("__final"); // internal for now
             printJSON(res);
         } else {
             notice("Downloaded '%s' to '%s' (hash '%s').",