Don't replay the gitian initial setup at each build

Summary:
This diff removes the gitian setup step from the build script. This step
can be performed once at installation.

Test Plan: Run the script on CI

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3709