being done in Prepare_config() meant that it would get saved into the
config stored on disk which is not what was intended.
being saved on disk is not a big problem, since the injection of the
dynamically determined option is always done on top of the value read
from the saved config anyway, so you'd always end up with two copies of
the option given to apt, the first being from the saved config, the second
being the more correct dynamically determined one, overriding the first.
so there is no functional change here, just better achieving what was
intended.
Gbp-Dch: Ignore
f4c8a28a21