cask/audit: Rework tmpdir removal

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.

Library/Homebrew/cask/audit.rb

@@ -47,12 +47,6 @@ 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!
@@ -540,6 +534,12 @@ module Cask
 
       @tmpdir ||= Pathname(Dir.mktmpdir("cask-audit", HOMEBREW_TEMP))
 
+      # Clean up tmp dir when @tmpdir object is destroyed
+      ObjectSpace.define_finalizer(
+        @tmpdir,
+        proc { FileUtils.remove_entry(@tmpdir) },
+      )
+
       ohai "Downloading and extracting artifacts"
 
       downloaded_path = download.fetch