[GITIAN] Don't ignore target-bin/

Summary:
This fixes an issue introduced by
https://github.com/devrandom/gitian-builder/pull/206.
The target-* pattern not only matches the target image name but also
the target-bin directory which contains useful files.

This allows for adding the missing files as well.

Depends on D6972.

Test Plan:

NOP()

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6973