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/tests/rspamd.nix

@@ -274,7 +274,10 @@ in
 
         I find cows to be evil don't you?
       '';
-      users.users.tester.password = "test";
+      users.users.tester = {
+        isNormalUser = true;
+        password = "test";
+      };
       services.postfix = {
         enable = true;
         destination = ["example.com"];