MacOS tasks can likely be made cheaper by creating a dedicated homebrew installation that's cached as a whole

[anarazel]

The homebrew installation for macos tasks takes 1min+ in each test run. We could instead do a separate homebrew install in a directory, and populate it from cache. Relveant .cirrus.yml piece:
https://github.com/postgres/postgres/blob/master/.cirrus.yml#L260

Docs for cirrus-ci cache stuff: https://cirrus-ci.org/guide/writing-tasks/#cache-instruction

[anarazel]

Example run taking 1:30 for homebrew install: https://cirrus-ci.com/task/4592442019151872?logs=homebrew_install#L0

[anarazel]

It still takes long, but the toolchain is different now on cirrus' side. VMs are used, but I don't think they can be provided externally as of now.