nix/modules/knot: block openssh password authentication

Signed-off-by: Seongmin Lee <[e-mail]>

nix/modules/knot.nix

···
           Match User ${cfg.gitUser}
               AuthorizedKeysCommand /etc/ssh/keyfetch_wrapper
               AuthorizedKeysCommandUser nobody
+              ChallengeResponseAuthentication no
+              PasswordAuthentication no
         '';
       };