cmake: on macOS fix dmg generation race condition

[jhoyt4]

On macOS 13, generation of the dmg distribution file intermidittenly
fails due to a "hdiutil: create failed - Resource busy" issue. Work
around this issue by creating a script that repeatedly attempts to
run hdiutil until success or 15 failures.

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Please review the checklist below to ensure that your contribution
to MythTV is ready for review by our developers.

It is helpful to regularly rebase your pull request to ensure changes
apply cleanly to the current MythTV development branch.

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[linuxdude42]

I still got a failure on the macos 13 builder.

[jhoyt4]

@linuxdude42 the failure looks to be unrelated.

The failure on the recent build was a runner/homebrew python3 install issue.

This PR should be unrelated as it only occurs if the code sign and distribution flags are manually set which are off by default in the CI chain.