renew_ra_cert: fix update of IPA RA user entry
The post-save hook for the RA Agent certificate invokes
cainstance.update_people_entry with the DER certificate instead of a
python-cryptograpy Certificate object. Apply to correct type.
Fixes: https://pagure.io/freeipa/issue/7282
Reviewed-By: Christian Heimes <cheimes@redhat.com>