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)