CheckFreeSpace always returns free space on system drive on Windows

In util.py, `CheckFreeSpace(path)` ignores the parameter on Windows, and returns `get_disk_free_space_ex(os.getenv('SystemDrive'))`.

When using cp and compressing data to be uploaded, gsutil writes data to a temporary directory. If the system drive is low on space, changing the temp directory to a different drive has no effect on the free space check.