Fixed whitespace issues in gitian-build.sh
Summary: Original file has an awful mix of tabs and spaces, making it completely unreadable.
Test Plan: Read it.
Reviewers: deadalnix, schancel, Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Subscribers: teamcity
Differential Revision: https://reviews.bitcoinabc.org/D2136