nixos/roon-server: fix broken user generation

Didn't notice this till I tried removing my custom roon user from the one I was testing with. There's not a 'groups' option for users, only group (primary group) and extraGroups. Use these.
 (#68337)

nixos/modules/services/audio/roon-server.nix

@@ -66,7 +66,8 @@ in {
       if cfg.user == "roon-server" then {
         isSystemUser = true;
         description = "Roon Server user";
-        groups = [ cfg.group "audio" ];
+        group = cfg.group;
+        extraGroups = [ "audio" ];
       }
       else {};
   };