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/pixiecore.nix

@@ -93,6 +93,7 @@ in
     users.users.pixiecore = {
       description = "Pixiecore daemon user";
       group = "pixiecore";
+      isSystemUser = true;
     };
 
     networking.firewall = mkIf cfg.openFirewall {