Problem:

 - Our branches based on devel use "latest" snapshots for every APT archive used
   at build time => their reproducibly_build_Tails_ISO_* job will fail if any of
   these APT snapshots is updated between the start of the original build job
   and the start of the reproducibly_build_Tails_ISO_* job.

 - Our branches based on stable are also affected, but to a lesser degree: they
   use the "latest" snapshot only for the debian-security archive.

 - Any branch can be affected when the build is triggered by a Git push at an
   unfortunate time. But for some branches, the automatic daily build is
   _always_ affected: daily Jenkins job runs are scheduled in a deterministic
   manner, with a schedule based on the name of the branch. So inevitably, the
   automatic daily rebuild of _some_ branches will always fail to build
   reproducibly, because the failure condition ("APT snapshots is updated
   betwee...