Correct the prior documented TOCTOU

Now, if a malicious validator set publishes a malicious `Batch` at the last
moment, it'll cause all future `Batch`s signed by the next validator set to
require a bool being set (yet they never will set it).

This will prevent the handover.

The only overhead is having two distinct `batch_message` calls on-chain.

substrate/client/tests/common/in_instructions.rs

@@ -39,7 +39,8 @@ pub async fn provide_batch(batch: Batch) -> [u8; 32] {
 
   let block = publish_tx(&Serai::execute_batch(SignedBatch {
     batch: batch.clone(),
-    signature: pair.sign(&batch_message(&batch)),
+    // TODO: This `batch.id == 0` line only works when session == 0
+    signature: pair.sign(&batch_message(batch.id == 0, &batch)),
   }))
   .await;