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/execution_layer/src/lib.rs

@@ -1393,12 +1393,13 @@ impl<T: EthSpec> ExecutionLayer<T> {
 
     pub async fn propose_blinded_beacon_block(
         &self,
+        block_root: Hash256,
         block: &SignedBeaconBlock<T, BlindedPayload<T>>,
     ) -> Result<ExecutionPayload<T>, Error> {
         debug!(
             self.log(),
             "Sending block to builder";
-            "root" => ?block.canonical_root(),
+            "root" => ?block_root,
         );
         if let Some(builder) = self.builder() {
             builder