* tool/downloader.rb (Downloader.save_cache): keep linked file
newer than cached file, so that GNU make triggers when the
content is updated. it uses the timestamp of symlink itself
instead of the target.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67413 b2dd03c8-39d4-4d8f-98ff-823fe69b080e