* tool/rbinstall.rb ($script_installer.stub): read stub file on
demand. as `$cmdtype` is set to "exe" in parse_args, it is not
set yet when `$script_installer` is defined.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63268 b2dd03c8-39d4-4d8f-98ff-823fe69b080e