brew: add contributed tap commands to PATH.

This means that taps root and `cmd` directories are added to the PATH.

This should enable migration of some of our contributed commands into
taps (e.g. `homebrew-boneyard`) and make it easy for third parties to
be able to maintain these. It might also make stuff easier for existing
tools like e.g. `brew-cask` and `boxen`.

Closes #32471.