Temporarily ignore debian.sources duing piuparts cleanup
Explicitly skip cleaning up /etc/apt/sources.list.d/debian.sources
in images/scripts/post_install_remove_configs
if it is present.
piuparts still needs to learn about debian.sources
, though at least if pipeline's own scripts don't delete debian.sources
then piuparts will not complain that it went away.
x-ref: !358 (comment 323457)