Unpack the overlay while downloading to avoid temporary storage

In some cases, like the `group-common-standalone-tpl.yaml` test set, the
overlay is quite big and causes out-of-space errors when `wget` tries to
save it in the tmpfs-backed `/tmp` directory.

To avoid that, use a pipe to unpack the tarball while `wget` streams it
on standard out, avoiding any intermediate storage.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>