CI: Set image to pureos/byzantium
Previously we relied on the default image set for the runner, which technically can be anything and isn't even guaranteed to include apt/dpkg.
Due to an influx of spam, we have had to impose restrictions on new accounts. Please see this page for instructions on how to get full permissions. Sorry for the inconvenience.
source.puri.sm migrated to a new server, see new ssh key fingerprint.
Previously we relied on the default image set for the runner, which technically can be anything and isn't even guaranteed to include apt/dpkg.