Avoid acquiring another read lock while holding one to avoid potential deadlock (#6200)

* Avoid acquiring another read lock to avoid potential deadlock.

beacon_node/eth1/src/service.rs

@@ -1129,7 +1129,7 @@ impl Service {
 
         Ok(BlockCacheUpdateOutcome {
             blocks_imported,
-            head_block_number: self.inner.block_cache.read().highest_block_number(),
+            head_block_number: block_cache.highest_block_number(),
         })
     }
 }