Partitioning is weird, but this seems to be a working configuration for
the Librem devices at least, that partman in d-i can accept and will not
try to override.
This is a hack, currently, which we need to ensure APT can fetch updates
in case we were unable to connect to the network while installing.
(if apt is not set up with sources, users will not know what to do at
all and assume the system is broken).