cask/audit: Rework tmpdir removal

[samford]

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I previously introduced a finalizer method in Cask::Audit to remove the created @tmpdir once it's no longer needed (#17358) but the existing approach produces a finalizer references object to be finalized warning when brew audit is run. I didn't see this warning when I was originally testing it but now it reliably appears.

This reworks the finalizer to define it within the #extract_artifacts method and use @tmpdir as the target object.

Fixes #17374.

[MikeMcQuaid]

Thanks for fix @samford. Would appreciate work on a better solution here that avoids the (very non-idiomatic) finaliser use.