bbl formula does not correctly select ARM binary for OSX

[ctlong]

brew install cloudfoundry/tap/bbl on a darwin/arm64 OS incorrectly installs the bbl binary for a darwin/amd64 OS.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. Unfortunately, the Pivotal Tracker project is private so you may be unable to view the contents of the story.

The labels on this github issue will be updated when the story is started.