Update mmdepbstrap and use gpgvnoexpkeysig
To replace the workaround we used in 1c5314b5 to solve the issue with expired jessie key, we can update mmdebstrap
(to 5b0bb46421ab946457b16d271bee0db08d9174a6 or later commit), and use mmdebstrap --aptopt='Apt::Key::gpgvcommand "/path/to/mmdebstrap/gpgvnoexpkeysig"'
.