copy: Use userauth-public-key/auto! for ssh authentification.

* guix/scripts/copy.scm (open-ssh-session): Replace userauth-agent! by
  userauth-public-key/auto!. This way, if ssh-agent is not run,
  default ssh key (~/.ssh/id_rsa) will be used as a fallback.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

guix/scripts/copy.scm

@@ -63,8 +63,8 @@ Throw an error on failure."
 
     (match (connect! session)
       ('ok
-       ;; Let the SSH agent authenticate us to the server.
+       ;; Use public key authentication, via the SSH agent if it's available.
-       (match (userauth-agent! session)
+       (match (userauth-public-key/auto! session)
          ('success
           session)
          (x