message: Remove unnecessary `mut` qualifier
diff: @@ -172,7 +172,7 @@ where
  // We create a new logging span, ensuring that we can always associate log messages to this
  // specific reactor. This is usually only relevant when running multiple reactors, e.g.
  // during testing, so we set the log level to `debug` here.
  - let mut span = tracing::debug_span!("node", id = tracing::field::Empty);
  + let span = tracing::debug_span!("node", id = tracing::field::Empty);
  let entered = span.enter();
  let event_size = mem::size_of::<R::Event>();
  