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/monitoring/tuptime.nix

@@ -34,7 +34,10 @@ in {
 
     users = {
       groups._tuptime.members = [ "_tuptime" ];
-      users._tuptime.description = "tuptime database owner";
+      users._tuptime = {
+        isSystemUser = true;
+        description = "tuptime database owner";
+      };
     };
 
     systemd = {