Bugfix: Do not hang until foregrounded when completed in background

On Linux, when running in the background restic failed to stop the terminal
output of the `backup` command after it had completed. This caused restic to
hang until moved to the foreground. This has now been fixed.

https://github.com/restic/restic/pull/3152
https://forum.restic.net/t/restic-alpine-container-cron-hangs-epoll-pwait/3334
