rbinstall.rb: respect DESTDIR

* lib/rubygems.rb (Gem.install): pass optinos to
  Gem::DependencyInstaller.

* tool/rbinstall.rb (gem): respect DESTDIR.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46823 b2dd03c8-39d4-4d8f-98ff-823fe69b080e