Use stable if matches! instead of if let match guard

`if let` guards are nightly-only (rust-lang/rust#51114), causing
`error[E0658]` and a CI `check-code` failure. Replace with the stable
`if matches!(...)` form suggested by rustc.

beacon_node/network/src/network_beacon_processor/mod.rs

@@ -983,8 +983,10 @@ impl<T: BeaconChainTypes> NetworkBeaconProcessor<T> {
                 );
             }
             Err(FetchEngineBlobError::BlobProcessingError(e))
-                if let BlockOrEnvelopeError::BlockError(BlockError::DuplicateFullyImported(..)) =
+                if matches!(
-                    *e =>
+                    *e,
+                    BlockOrEnvelopeError::BlockError(BlockError::DuplicateFullyImported(..))
+                ) =>
             {
                 debug!(
                     %block_root,