Bugfix: Correctly detect output redirection for `backup` command on Windows

On Windows, since restic 0.10.0 the `backup` command did not properly detect
when the output was redirected to a file. This caused restic to output
terminal control characters. This has been fixed by correcting the terminal
detection.

https://github.com/restic/restic/issues/3111
https://github.com/restic/restic/pull/3150