Clean up cask audit tmpdir after use

`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.

Library/Homebrew/cask/audit.rb

@@ -47,6 +47,12 @@ module Cask
       @token_conflicts = token_conflicts
       @only = only || []
       @except = except || []
+
+      # Clean up `#extract_artifacts` tmp dir when Audit object is destroyed
+      ObjectSpace.define_finalizer(
+        self,
+        proc { FileUtils.remove_entry(@tmpdir) if @tmpdir },
+      )
     end
 
     def run!