rbinstall.rb: CONFIG['arch'] not $arch

* tool/rbinstall.rb: use CONFIG['arch'] instead of $arch which was not
  added finally.  fix r39108.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39131 b2dd03c8-39d4-4d8f-98ff-823fe69b080e