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/databases/pgmanage.nix

@@ -197,6 +197,7 @@ in {
         group = pgmanage;
         home  = cfg.sqlRoot;
         createHome = true;
+        isSystemUser = true;
       };
       groups.${pgmanage} = {
         name = pgmanage;