[stable] GHA: Switch from macos-latest (currently *arm64* macOS 14) to macOS 12

[kinke]

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[kinke]

Okay, macos-12 works; I'll try macos-13 shortly too.

[kinke]

Nope, same problem (Mac linker assertion) on macOS 13 x86_64. I guess it's Xcode v15 then, as both macos-{13,14} images currently default to v15.0.1, while macos-12 defaults to v14.2.

https://github.com/dlang/installer/actions/runs/9025225631/job/24800683396?pr=576

[ibuclaw]

Ok, I'll merge this as-is for now as I see everything is green. Investigating Xcode 15 can then be done separately.