flatdeb-builder: Fix test by not hardcoding the release name

Rather than hardcoding the release name in the filename of the YAML
definition of the APT suite, generate it at runtime so that we can
also inherit the osname and APT mirror from the main settings, which
is useful for downstreams.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>