limit-upload does not work for rclone backend

[fd0]

Discovered by @julianbrost: The rclone backend does not respect --limit-upload. Oftentimes it's not necessary to use this, as the limiting in rclone itself should be used because it gives much better results, but in case a remote instance of rclone is used (e.g. called via ssh), it is still relevant to limit the bandwidth from restic to rclone.