[CI] Make gitian builds run on multiple cpus

Summary:
Enable the -j option when running gbuild. The memory is set to 3.5GB
because our machines have 4GB RAM.

Test Plan: Run the nightly builds on CI

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2893