Merge #21112: ci: use Focal for macOS cross builds

ac24af453d16afd1993fa1f292aa41ae6b16f138 ci: use Ubuntu Focal for macOS cross build (fanquake)

Pull request description:

  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:
  ```bash
  Running 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.

ACKs for top commit:
  MarcoFalke:
    cr ACK ac24af453d16afd1993fa1f292aa41ae6b16f138

Tree-SHA512: 587ba5acf741bcefecf1bc262fa1177f565ebfa9de56125eca19ed3c7db7b9aabfb96866e9c140681b88cb7015a3ded2bc6b4b1b235543d6f6e9dfc6984d569f

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