Retry if DMG BOM is empty.

[reitermarkus]

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This error is pretty much impossible to reproduce and only happens rarely on CI.

My naive guess is that mounting a DMG on CI is slow sometimes meaning we try finding files which aren't mounted yet. Not sure if that's actually how hdiutil works but we'll see.

Fail in both cases, so we actually see if retrying fixes this or not.

[BrewTestBot]

Review period will end on 2023-03-14 at 17:34:18 UTC.

[p-linnane]

Every time I've seen this happen it's fixed by a rerun, so this would be useful.

[MikeMcQuaid]

Makes sense, thanks @reitermarkus!

[BrewTestBot]

Review period skipped due to critical label.