bin/phase-helpers.shL Fix unpack() for deb2targz .xz types

Issue reported by: mva in irc

>>> Unpacking source...
>>> Unpacking 2gis_4.14.11-0trusty1+shv139+r191_amd64.deb to /var/tmp/portage/app-misc/2gis-4.14.11.139.191/work
deb2targz: converting '2gis_4.14.11-0trusty1+shv139+r191_amd64.deb' ...
deb2targz: skipping section 'debian-binary'
deb2targz: skipping section 'control.tar.gz'
deb2targz: wrote '2gis_4.14.11-0trusty1+shv139+r191_amd64.tar.xz'
mv: cannot stat ‘2gis_4.14.11-0trusty1+shv139+r191_amd64.tar.gz’: No such file or directory