v2021pre: backport from v2022dev0: Unpack the overlay while downloading to avoid temporary storage

Backport of !221 (merged)

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.