- Explain why `HOMEBREW_UPDATE_TEST` is set and what it does.
- Avoid auto-updating (this should already be not happening but let's
make sure).
- Set environment variables to ensure we're testing the default update
behaviour (rather than deferring to user configuration).
- Use `HOMEBREW_BREW_GIT_REMOTE` as well as setting
`git config remote.origin.url` ourselves.
- Run `brew help` quietly first to hide irrelevant portable Ruby output.
- Run `brew update --verbose --debug` to get even more output on
failure.
Fixes https://github.com/Homebrew/brew/issues/8979
878cbb634f