The VirtualBox VM is running tight on available space with nearly 100% usage.
This frees things up a bit by cleaning the apt cache after Gitian installation.
Don't use depth: 1 or ignore_errors in git tasks.
Commit history is nice, and since you can't do most things without the
repositories, we should fail closed upon errors.
Creates variable for 'gitian_builder_url', so we can switch out @ageis's fork
with the official @devrandom repo once a pending pull request is merged.
Changes default zcash_version to 'master' in defaults/main.yml.
Kevin Gallagher