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.

• Failing job here: https://lava.collabora.co.uk/scheduler/job/2835952
• Patched test job here: https://lava.collabora.co.uk/scheduler/job/2842189