rubygems calls chdir directly, actions should be enclosed in a block
When installing gems, windows_stub_script is called in installer.rb which
directly calls Dir.chdir.
If this were to be called inside another Dir.chdir call with a block passed to
it, it would spew warnings.
Instead the proposed patch encloses the actions in a block, removing the need
for saving and restoring a path around the chdir call.