rbinstall.rb: fix bundled gems location

* rbinstall.rb: fix target location for installing bundled gems.
  install to the prepared directory instead of default Gem.dir,
  not to be affected GEM_HOME environment variable.  [Fix GH-798]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49047 b2dd03c8-39d4-4d8f-98ff-823fe69b080e