nixos/resilio: set rslsync gid (#350055)

nixos/modules/services/networking/resilio.nix

···
       group           = "rslsync";
     };
 
-    users.groups.rslsync = {};
+    users.groups.rslsync.gid = config.ids.gids.rslsync;
 
     systemd.services.resilio = with pkgs; {
       description = "Resilio Sync Service";