Pull back code to set up GOPATH for build

If there’s no GOPATH set up locally, Go won’t be able to find sub packages. This is due to changes in https://github.com/github/hub/commit/af9163b699b34ffa167ef99853ff2f7374d7b01a.

This is to fix https://github.com/Homebrew/homebrew/pull/33407#issuecomment-65016107