Added missing instructions for OSX gitian building
Summary:
Similar instructions exist in doc/gitian-building/gitian-building-max-os-sdk.md but these
changes are more tailored to our current VM setup instructions. They probably need to be de-duped at some point.
Test Plan:
- Follow all setup instructions, including with these changes.
- ./gitian-build.py -c -b -o m -n gitianuser v0.20.6
Reviewers: #bitcoin_abc, deadalnix, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D4435