bs_published_hook_aptly_snapshot: Generate latest metadata

The latest published snapshot required latest/ dir and latest.txt file
to point to it. Generate those.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
4 jobs for wip/adalessandro/aptly-snapshot-prefixdistro in 27 minutes and 45 seconds (queued for 1 minute and 35 seconds)