CPack: Fix .dmg HFS+ creation on macOS APFS hosts

Brad Kingrequested to merge
brad.king/cmake:cpack-dmg-explicit-hfs into master

When running hdiutil create, specify the HFS+ filesystem explicitly. Otherwise hdiutil may choose a filesystem based on the host. We do not want to create APFS images for .dmg packages because they may not mount on macOS versions prior to 10.12.

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