gpg: Fix another regression due to the T7547 fix.
* g10/getkey.c (get_pubkey_for_sig): Keep a requested PUBKEY_USAGE_CERT. (finish_lookup): For correctness in future use cases allow PUBKEY_USAGE_CERT to also trigger verify mode.
The case here was that a cert-only primary key was removed with
export-clean.
- GnuPG-bug-id: T7583