Limit progress bar updates to once per second on non-terminal outputs

The code accidentally checked whether stdin is a terminal instead of
stdout, the former is not relevant here as the output is printed on
stdout.

cmd/restic/progress.go

@@ -16,7 +16,7 @@ func newProgressMax(show bool, max uint64, description string) *progress.Counter
 	}
 
 	interval := time.Second / 60
-	if !stdinIsTerminal() {
+	if !stdoutIsTerminal() {
 		interval = time.Second
 	} else {
 		fps, err := strconv.ParseInt(os.Getenv("RESTIC_PROGRESS_FPS"), 10, 64)