`Cask::Audit#extract_artifacts` is used in the `#audit_signing` and
`#cask_plist_min_os` methods to create a directory in `/tmp` and
extract cask artifacts without duplicating the work if it's already
done. However, due to how this is set up, `tmpdir` isn't removed
afterward and the extracted artifacts will take up disk space until
the `tmp` directory is cleaned up. As a result, running
`brew audit --strict --online` locally can chew through disk space
and it may not be clear to the user where their free space has gone.
This adds a finalizer method to `Cask::Audit` to remove the created
`@tmpdir` (if any) once it's no longer needed. There may be a better
way of addressing the issue but this works for now without having to
restructure how these audits work.
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