Sure, but `Dir.mktmpdir` creates it. And it would only remove the directory if we passed a block to it ([docs](http://www.ruby-doc.org/stdlib-1.9.3/libdoc/tmpdir/rdoc/Dir.html#method-c-mktmpdir). But in this case we obviously can't. (Well, we *could* pass a non-op block to remove the dir as a side effect. But `Berksfile#vendor` will anyway create it again.)