swap unnecessary write lock to read lock in block_verification (#4340)

## Issue Addressed

[#4334](https://github.com/sigp/lighthouse/issues/4334)

## Proposed Changes

swap unnecessary write lock to read lock

## Additional Info

N/A


Co-authored-by: Michael Sproul <micsproul@gmail.com>

beacon_node/beacon_chain/src/block_verification.rs

@@ -747,7 +747,7 @@ impl<T: BeaconChainTypes> GossipVerifiedBlock<T> {
         // We check this *before* we load the parent so that we can return a more detailed error.
         check_block_is_finalized_checkpoint_or_descendant(
             chain,
-            &chain.canonical_head.fork_choice_write_lock(),
+            &chain.canonical_head.fork_choice_read_lock(),
             &block,
         )?;