Manually add ffmpeg and ffprobe

[whyboris]

😓 worse in some ways, but allows for manually choosing which version of FFmpeg to include 🤔

Currently Mac builds are broken with this -- because Mac complains about the security of executing a file that's already part of the build package :trollface:

And build sizes are double of what they were before 😓 -- so probably not worth merging this in (unless I figure out how to bundle things better) 😓