Clean up cask audit tmpdir after use

[samford]

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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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As discussed on Slack, it probably makes sense for brew cleanup to also clean up tmp directories that brew has created. From a quick glance at our use of #mktmpdir, we would probably have to institute a common prefix (e.g., using homebrew- or homebrew-brew- before existing prefixes) to be able to reasonably identify brew-created tmp directories (without manually maintaining a list of prefixes in use). There may be a better way of going about it but that was my initial thought.

That said, it may not be necessary if everything's working correctly. With this patch in place, I've only ended up with a lingering homebrew-unpack tmp directory on maybe one or two occasions but I haven't figured out why that happened (my best guess is that I may have prematurely interrupted a command using ^C).

[MikeMcQuaid]

As discussed on Slack, it probably makes sense for brew cleanup to also clean up tmp directories that brew has created. From a quick glance at our use of #mktmpdir, we would probably have to institute a common prefix (e.g., using homebrew- or homebrew-brew- before existing prefixes) to be able to reasonably identify brew-created tmp directories (without manually maintaining a list of prefixes in use). There may be a better way of going about it but that was my initial thought.

This is a good idea 👍🏻

(my best guess is that I may have prematurely interrupted a command using ^C).

I think we should still endeavour to have brew cleanup handle these.

[MikeMcQuaid]

Thanks for PR! Approach seems fine for now to fix the bug but I think ideally this would have the tmpdir usage be all within one block so cleanup happens as typical at the end of the block.