Add enable custom environment path for brew

[juanjsebgarcia]

This is a small fix that allows a custom path to be declared for brew.

It is common to have brew in a non-default location when the system is multi-user, as it keeps packages seperate from other users on the machine.

The code sample checks for an environment variable called BREW_BIN and uses that, falling back to the existing logic if it does not exist.

[sergeycherepanov]

Thanks!