Part of #163896.

Today, bin/internal/engine.version is the only untracked file in bin/internal, and is overloaded to mean "the version of the engine we've computed" and "woah, you've tracked this file, so that means this is a release branch, so let's just use this file".

That is confusing, and makes git unhappy (as the file will go from tracked <> untracked as you switch channels or branches). Instead, we should make the stamped (current) engine version bin/cache/engine.stamp (for consistency), and make bin/internal/engine.version tracked, but optional (and if it's checked-in, we use it).