Bugfix: Print `created new cache` message only on a terminal

The message `created new cache` was printed even when the output wasn't a
terminal. That broke piping `restic dump` output to tar or zip if cache
directory didn't exist. The message is now only printed on a terminal.

https://github.com/restic/restic/issues/3334
https://github.com/restic/restic/pull/3343
