agent: Make sure to return success in ephemeral store mode.

* agent/genkey.c (store_key): Clear ERR on success.

This fixes a real problem which might let ephemeral store mode fail
randomly.

This is part of

(cherry picked from commit fdc5003956407da1984f40fc27115e4704587e15)

• GnuPG-bug-id: T7129
• Co-authored-by: Jakub Jelen <jjelen@redhat.com>