When using --standalone, place the setup.rb file relative to the gem install path, rather than the working directory

For issue #1643 I am running bundler from the root of my project. There are several subdirectories with their own Gemfiles. For each subdirectory, I run:

  bundle install --gemfile=path/to/Gemfile --standalone

This installs the gems into path/to/vendor/bundle/ruby.

However the setup.rb file is created at vendor/bundle/bundler/setup.rb, in the project root. I would like this file to be located at path/to/vendor/bundle/bundler/setup.rb. This patch provides the desired behavior. Thanks!