I previously introduced a finalizer method in `Cask::Audit` to remove
the created `@tmpdir` once it's no longer needed 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.
e2220ecc42