[Gitian] don't untar the source package

Summary: Because we don't build from the source package, there is no reason to untar the archive for every host. Instead we use a different build directory for each host.

Test Plan: Run the gitian builds.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13452