CPack: Fix PackageMaker .dmg HFS+ creation on macOS APFS hosts
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. This was missed in commit 39b50975 (CPack: Fix .dmg HFS+ creation on macOS APFS hosts, 2017-10-02). Reported-by: Alan Garny