Previously, the only way to ensure a clean release tarball was to
start from scratch, fetching all the contrib sources from the
network (> 1 GiB).  This change leverages the new 'list-tarballs'
targets of the contrib build system to allow reusing the relevant
tarballs from the cache when available, which translates to faster
builds.

It also constructs the pristine source release archive via
git-archive, which guards against including unwanted files.  Since
only the required tarballs are included in the source archive, its
size is reduced from 1.4 GiB to 72 MiB.

GitLab: jami-packaging#55
Change-Id: I4993b269f3b97f6a4b6b8592aa8b5f4eb5448943