Actually use the upstream tarball so the upload is fine

Retrieve the ../fossology_3.1.orig.tar.gz tarball from the swh debian repository (so the
checksums match during the upload step).

When built from source, the checksums actually mistmatch during upload.

Ideally we should use pristine-tar (see next commit)

Related to T3039

debian/gbp.conf

 [DEFAULT]
 upstream-branch=master
-upstream-tree=master
+git-tarball-dir=..
 debian-branch=debian/buster-swh
 pristine-tar=False

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T3039 is now swh/infra/sysadm-environment#3039 (closed)