brew: Make HOMEBREW_LIBRARY based off of HOMEBREW_REPOSITORY
brew: Make HOMEBREW_LIBRARY based off of HOMEBREW_REPOSITORY

This makes the logic match the Ruby code from before it was moved here
(see 390e327f55126f7ee9519bc6c03a0c0aa14f7168)

filcab/homebrew,filcab/homebrew,filcab/homebrew,filcab/homebrew