If --cache-dir is passed in as a relative path, then rclone will not
be able to turn it into a UNC path under Windows, which means that
file names longer than 260 chars will fail when stored in the cache.
This patch makes the --cache-dir path absolute before using it.
See: https://forum.rclone.org/t/handling-of-long-paths-on-windows-260-characters/20913