ci: wasted time in brew_install stage of macOS native build #22068

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?

The images are immutable by it's nature and we don't usually update things for xcode versions of the images. We do update the base versions periodically. Is it ok to build with Xcode 12.5? big-sur-xcode-12.5 was built on April 29th and should have brew formulas not pointing to bintray.