attributes: extract match scrutinee in err-only case

A large match scrutinee was causing clippy warnings. PR #2880
fixed this for the case where both err and ret are present,
but the exact same issue occurs with just err present, and that
has not been fixed.

This pull request applies the same fix to the err-only case.

tracing-attributes/src/expand.rs

@@ -293,7 +293,8 @@ fn gen_block<B: ToTokens>(
             ),
             (Some(err_event), None) => quote_spanned!(block.span()=>
                 async move {
-                    match async move #block.await {
+                    let __match_scrutinee = async move #block.await;
+                    match  __match_scrutinee {
                         #[allow(clippy::unit_arg)]
                         Ok(x) => Ok(x),
                         Err(e) => {