CI: Use Xcode 12.4 for building Apple things (#870)
CI: Use Xcode 12.4 for building Apple things (#870)

It seems 12.0 is broken somehow on CI, let's upgrade! #YOLO

In any case, Apple does not support anything but the latest version,
and GitHub Actions will soon be ready to support macOS 11 for non-paying
plebs and we will have to upgrade something somewhere eventually too.
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