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/bird.nix

@@ -73,6 +73,7 @@ let
           users.${variant} = {
             description = "BIRD Internet Routing Daemon user";
             group = variant;
+            isSystemUser = true;
           };
           groups.${variant} = {};
         };