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/misc/bazarr.nix

@@ -64,6 +64,7 @@ in
 
     users.users = mkIf (cfg.user == "bazarr") {
       bazarr = {
+        isSystemUser = true;
         group = cfg.group;
         home = "/var/lib/${config.systemd.services.bazarr.serviceConfig.StateDirectory}";
       };