CI: Remove optimistic continuation of disk image creation for macOS

The situation of the mount point being unmounted after an unsuccessful
detach attempt leads to an unrecoverable situation. Instead of
optimistically continuing, the script has to fail.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f