I had assumed Cirrus was spinning up Docker containers to run the CI,
however we are actaully running on the Cirrus machines themselves. See
DANGER_RUN_CI_ON_HOST
and in the logs:
0Running on host system without docker wrapper
So with this change we will actually be using Focal for the macOS cross build.
Follow up to #21036.
This originally contained Windows changes, and an attempt to get Cirrus running without DANGER_RUN_CI_ON_HOST
, however that seems non-trival, so Windows changes have been dropped from here for now.