reproducible_debian_live_build: Extend testing

Extend the tests to include bookworm and sid.

Reduce the testing frequency for bullseye to monthly.

Document each hook and turn it off when the Debian version is
sufficiently new.

Do not use the freshly built ISO files as artifact. They would be able
to consume too much hard disc space in case of differences.

Signed-off-by: Holger Levsen <holger@layer-acht.org>