boring-sys: Don't use CMake cross-compilation for macOS->iOS #636

Job	Run time
clippy	2m 0s
Cross build from macOS to Linux (x86_64-unknown-linux-gnu)	3m 53s
rustfmt	4s
Cross build from macOS to Linux (FIPS) (x86_64-unknown-linux-gnu)	5m 44s
Test features	3m 38s
Test (stable)	2m 26s
Test FIPS integration	3m 42s
Test (macos-x86_64)	4m 9s
Test (arm-android)	2m 59s
Test (arm64-android)	2m 20s
Test (i686-android)	2m 33s
Test (x86_64-android)	2m 43s
Test (aarch64-ios)	3m 30s
Test (aarch64-ios-sim)	5m 4s
Test (x86_64-ios)	4m 57s
Test (i686-linux)	2m 1s
Test (arm-linux)	3m 37s
Test (aarch64-linux)	3m 25s
Test (x86_64-musl)	2m 59s
Test (x86_64-mingw)	2m 23s
Test (i686-msvc)	7m 10s
Test (x86_64-msvc)	5m 27s
	1h 16m 44s

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