CPack: Fix .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.
4 jobs for cpack-dmg-explicit-hfs
Status Name Job ID Coverage
  External
passed ghostflow-branch-check-master

passed ghostflow-commit-check

passed ghostflow-stager

passed ghostflow-stager