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