Bugfix: `backup --quiet` no longer prints status information

A regression in the latest restic version caused the output of `backup --quiet`
to contain large amounts of backup progress information when run using an
interactive terminal. This is fixed now.

A workaround for this bug is to run restic as follows:
`restic backup --quiet [..] | cat -`.

https://github.com/restic/restic/issues/3184
https://github.com/restic/restic/pull/3186