Don't create a execution payload with same timestamp as terminal block (#3331)

## Issue Addressed

Resolves #3316 

## Proposed Changes

This PR fixes an issue where lighthouse created a transition block with `block.execution_payload().timestamp == terminal_block.timestamp` if the terminal block was created at the slot boundary.

beacon_node/execution_layer/src/engine_api.rs

@@ -106,6 +106,8 @@ pub struct ExecutionBlock {
     pub block_number: u64,
     pub parent_hash: ExecutionBlockHash,
     pub total_difficulty: Uint256,
+    #[serde(with = "eth2_serde_utils::u64_hex_be")]
+    pub timestamp: u64,
 }
 
 /// Representation of an exection block with enough detail to reconstruct a payload.