Ever since #21663, which added a brew update command, >10% of the runtime of the macOS native CI is spent repetitively doing this update. For example, in this recent build, https://cirrus-ci.com/task/4631533593034752, the brew_install stage takes 8m26s to complete..

This seems like something we should be able to avoid. @fkorotkov is it possible for the CI images to be updated to contain a more up to date version of brew or similar?