nixos/dnscache: Provide explicit group for "dnscache" user

Without this patch the module refuses to evaluate, complaining that
default "nogroup" is no longer supported.

nixos/modules/services/networking/dnscache.nix

@@ -86,7 +86,11 @@ in {
 
   config = mkIf config.services.dnscache.enable {
     environment.systemPackages = [ pkgs.djbdns ];
-    users.users.dnscache.isSystemUser = true;
+    users.users.dnscache = {
+        isSystemUser = true;
+        group = "dnscache";
+    };
+    users.groups.dnscache = {};
 
     systemd.services.dnscache = {
       description = "djbdns dnscache server";