CI: ensure that macOS runner is x64 #5755

Job	Run time
Swift on MacOS	9m 21s
Dart with AddressSanitizer	3m 13s
Swift on Linux	4m 1s
Android-Kotlin	10m 11s
CMake toolchain tests for Android	6m 18s
Dart	4m 59s
Android	6m 5s
C++	2m 19s
	46m 27s

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