Do not pull from Core repos when using gitian-build.sh
Summary:
Minor security risk pulling from untrusted sources.
Depends on D2052
Test Plan: None for now. This script needs more fixing before it works with our release process.
Reviewers: deadalnix, schancel, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Subscribers: teamcity
Differential Revision: https://reviews.bitcoinabc.org/D2055