message: subscriber: fix clippy needless borrow warning
  This fixes CI on 1.57.0.
diff: @@ -341,7 +341,7 @@ pub(in crate::field) mod test_util {
  impl<'a> Visit for DebugVisitor<'a> {
  fn record_debug(&mut self, field: &Field, value: &dyn fmt::Debug) {
  - write!(&mut self.writer, "{}={:?}", field, value).unwrap();
  + write!(self.writer, "{}={:?}", field, value).unwrap();
  }
  }
  