It's not being generated on Linux. I'm pretty sure this is a bug (or at
least undocumented behavior), but it's OK because Linux doesn't support
updates yet anyway.
These are the assets that go in s3://releases.lbry.io/app/latest.
Included are the built asset used for updates (on Mac this is actually
a zip file, not the dmg), as well as the update metadata file for this
platform (latest.yml, etc.)
Also includes some refactoring in the update build script.