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
4 jobs for cpack-dmg-explicit-hfs