add support for debug checker log output

With monzero supporting slog to get debug output from check commands,
moncheck now supports that too by changing the level to debug.

cmd/moncheck/main.go

@@ -91,6 +91,7 @@ func main() {
 		Timeout:        timeout,
 		HostIdentifier: hostname,
 		Executor:       monzero.CheckExec,
+		Logger:         logger,
 	})
 	if err != nil {
 		logger.Error("could not create checker instance", "error", err)