Fix the isolated environment spec on OS X

The reason this spec was failing is that on OS X the temp directory is
actually something like this:

    /var/folders/bg/8sxn6p0j5kv0j1z__3hwz5nm0000gn/T/

However, /var is not an actual folder, but a symlink to /private/var and
that causes the check for root_level in `ConfigLoader.dirs_to_search()` to
fail and the ascend to continue.