ci: macos: set the cross compile cmake option
ci: macos: set the cross compile cmake option

This should avoid invoking llvm-config to work around a symbol
not found inside libstdc++, and plus it's technically more correct
anyway.

zig-lang/zig,raulgrell/zig,andrewrk/zig,andrewrk/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,zig-lang/zig