Allow the ability for a system administrator to use
`HOMEBREW_BREW_WRAPPER` and `HOMEBREW_FORCE_BREW_WRAPPER` variables to
enforce the usage of a particular `brew` command for non-trivial (e.g.
`brew --prefix` is considered trivial, it doesn't need to write to the
prefix) Homebrew commands.
This also introduces a `HOMEBREW_ORIGINAL_BREW_FILE` variable for some
internal usage; `HOMEBREW_BREW_FILE` was being used internally for
both "how should we shell out to Homebrew" and "what should we use
to check permissions on Homebrew". `HOMEBREW_ORIGINAL_BREW_FILE` is
now used just for the latter case.
Inspired by conversation in
https://github.com/Homebrew/homebrew-bundle/pull/1551 which suggested
this was worth fixing in wider than just `brew bundle`.
e9b4979f40