Deduplicate block root computation (#3590)

## Issue Addressed

NA

## Proposed Changes

This PR removes duplicated block root computation.

Computing the `SignedBeaconBlock::canonical_root` has become more expensive since the merge as we need to compute the merke root of each transaction inside an `ExecutionPayload`.

Computing the root for [a mainnet block](https://beaconcha.in/slot/4704236) is taking ~10ms on my i7-8700K CPU @ 3.70GHz (no sha extensions). Given that our median seen-to-imported time for blocks is presently 300-400ms, removing a few duplicated block roots (~30ms) could represent an easy 10% improvement. When we consider that the seen-to-imported times include operations *after* the block has been placed in the early attester cache, we could expect the 30ms to be more significant WRT our seen-to-attestable times.

## Additional Info

NA

beacon_node/network/src/beacon_processor/tests.rs

@@ -242,6 +242,7 @@ impl TestRig {
 
     pub fn enqueue_rpc_block(&self) {
         let event = WorkEvent::rpc_beacon_block(
+            self.next_block.canonical_root(),
             self.next_block.clone(),
             std::time::Duration::default(),
             BlockProcessType::ParentLookup {
@@ -253,6 +254,7 @@ impl TestRig {
 
     pub fn enqueue_single_lookup_rpc_block(&self) {
         let event = WorkEvent::rpc_beacon_block(
+            self.next_block.canonical_root(),
             self.next_block.clone(),
             std::time::Duration::default(),
             BlockProcessType::SingleBlock { id: 1 },