nixos/users: require one of users.users.name.{isSystemUser,isNormalUser}

As the only consequence of isSystemUser is that if the uid is null then
it's allocated below 500, if a user has uid = something below 500 then
we don't require isSystemUser to be set.

Motivation: https://github.com/NixOS/nixpkgs/issues/112647

nixos/modules/services/networking/ncdns.nix

@@ -243,8 +243,10 @@ in
         xlog.journal = true;
     };
 
-    users.users.ncdns =
-      { description = "ncdns daemon user"; };
+    users.users.ncdns = {
+      isSystemUser = true;
+      description = "ncdns daemon user";
+    };
 
     systemd.services.ncdns = {
       description = "ncdns daemon";