remove retry check from signature validation

The retry path was removed by
4ccf16dff589ce9f369d377bb5d3f490bd27c624

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
check_keyring: skip keys known to be missing

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
_alpm_key_in_keychain: cache known keys

Implements FS#38042

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>