Do this instead of appending the `HOMEBREW_PREFIX` as it's more reliable at finding e.g. keg-only and requirement PATH additions. It's not ideal as it overrides some environment filtering but the only real alternative is recreate the entire build environment by shelling out to `build.rb` in `formula_installer`.
Library
This directory contains all the code run by the official brew and brew cask commands in Homebrew and all formulae (package descriptions) in taps (repositories containing formulae) in Taps subdirectories.