Bugfix: Prevent `backup --stdin-from-command` from hanging

When using `--stdin-from-command`, the `backup` command could hang until
manually cancelled if the backup stopped before all subprocess output was
consumed, for example after a failed upload to the repository. This has
now been fixed.

https://github.com/restic/restic/issues/5683
https://github.com/restic/restic/pull/21829
