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/backup/borgbackup.nix

@@ -169,6 +169,7 @@ let
         (map (mkAuthorizedKey cfg false) cfg.authorizedKeys
         ++ map (mkAuthorizedKey cfg true) cfg.authorizedKeysAppendOnly);
       useDefaultShell = true;
+      isSystemUser = true;
     };
     groups.${cfg.group} = { };
   };