message: ci: Disable homebrew auto-updating
  This can introduce flakiness when GitHub returns error codes. The VMs'
  brew environments should be up to date "enough" anyways.
diff: # https://github.com/actions/virtual-environments/blob/master/images/macos/macos-10.15-Readme.md for
  # a list of pre-installed tools in the macOS image.
  +export HOMEBREW_NO_AUTO_UPDATE=1
  +
  function is_installed {
  brew ls --versions "$1" >/dev/null
  }
  