nixos/zigbee2mqtt: run as zigbee2mqtt group

Not setting a group is a security defect, since that will run the unit
under the root group.

Fixes: 1af87596 ("nixos/zigbee2mqtt: init")

nixos/modules/services/misc/zigbee2mqtt.nix

@@ -79,6 +79,7 @@ in
       serviceConfig = {
         ExecStart = "${cfg.package}/bin/zigbee2mqtt";
         User = "zigbee2mqtt";
+        Group = "zigbee2mqtt";
         WorkingDirectory = cfg.dataDir;
         Restart = "on-failure";