Merge pull request #5542 from samueldr/fix/registry-experimental

nix registry: Mark experimental

src/nix/registry.cc

@@ -226,6 +226,7 @@ struct CmdRegistry : virtual NixMultiCommand
 
     void run() override
     {
+        settings.requireExperimentalFeature(Xp::Flakes);
         if (!command)
             throw UsageError("'nix registry' requires a sub-command.");
         command->second->prepare();