Start recording git versions and formula versions in builds, for repeatability

[apjanke]

The build is sensitive to the versions / state of stuff in relted repos, especially the homebrew core formulae/tap and homebrew-octave-app Tap repos. Should capture the git commit hash of each relevant repo, along with the versions of all the relevant formulae, in a file somewhere, and preserve that. Maybe inside the app build itself, maybe in the octave-app or octave-app-bundler repo, maybe both.

Then maybe we could do more-reproducible builds by checking out the tap repos to that same commit, to get the older versions of the dependency packages and formulae/build instructions.