nixos/unifi: define group, fix after #133166

nixos/modules/misc/ids.nix

@@ -215,7 +215,7 @@ in
       ntp = 179;
       zabbix = 180;
       #redis = 181; removed 2018-01-03
-      unifi = 183;
+      #unifi = 183; dynamically allocated as of 2021-09-17
       uptimed = 184;
       zope2 = 185;
       ripple-data-api = 186;

nixos/modules/services/networking/unifi.nix

@@ -115,10 +115,12 @@ in
   config = mkIf cfg.enable {
 
     users.users.unifi = {
-      uid = config.ids.uids.unifi;
+      isSystemUser = true;
+      group = "unifi";
       description = "UniFi controller daemon user";
       home = "${stateDir}";
     };
+    users.groups.unifi = {};
 
     networking.firewall = mkIf cfg.openPorts {
       # https://help.ubnt.com/hc/en-us/articles/218506997