Tests/RunCMake/XcFramework: Use macosx SDK in all multi-arch macOS cases

We select the list of macOS architectures based on the version of Xcode.
Therefore we should use a macosx SDK that comes with Xcode to make sure
it has all the selected architectures.  For example, on macOS 10.15 the
system `/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk` SDK
may have only `x86_64`, but Xcode 12 can be installed with SDKs that
support both `arm64` and `x86_64`.