Bugfix: Allow `rclone` and `sftp` backends when running in background

Previously, starting restic in the background could result in unexpected
behavior when using the `rclone` or `sftp` backends. For example,
`restic -r rclone:./example --insecure-no-password init &` could cause the
calling `bash` shell to exit unexpectedly.

This has now been fixed.

https://github.com/restic/restic/issues/5354
https://github.com/restic/restic/pull/5358
https://github.com/restic/restic/pull/5493
https://github.com/restic/restic/pull/5494
