message: make just ./miri print help text without 'unknown command'
diff: @@ -141,8 +141,10 @@ run|run-debug)
  exec cargo run $CARGO_BUILD_FLAGS -- --sysroot "$MIRI_SYSROOT" "$@"
  ;;
  *)
  + if [ -n "$COMMAND" ]; then
  echo "Unknown command: $COMMAND"
  echo
  + fi
  echo "$USAGE"
  exit 1
  esac
  