[CMAKE] Don't distribute gitian as part of our sources package

Summary:
It's not needed to build Bitcoin ABC.

Depends on D6973.

Test Plan:

ninja package_source

Check the gitian-builder directory is not part of the sources archive.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6974