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.

tests/coordinator/src/tests/batch.rs

@@ -165,7 +165,7 @@ pub async fn batch(
   let signature = Signature(
     schnorrkel::keys::Keypair::from_bytes(&schnorrkel_key_pair)
       .unwrap()
-      .sign_simple(b"substrate", &batch_message(&batch))
+      .sign_simple(b"substrate", &batch_message(batch.id == 0, &batch))
       .to_bytes(),
   );
 

tests/processor/src/tests/batch.rs

@@ -137,8 +137,10 @@ pub(crate) async fn sign_batch(
           messages::substrate::ProcessorMessage::SignedBatch { batch: this_batch },
         ) => {
           if batch.is_none() {
-            assert!(PublicKey::from_raw(key)
-              .verify(&batch_message(&this_batch.batch), &this_batch.signature));
+            assert!(PublicKey::from_raw(key).verify(
+              &batch_message(this_batch.batch.id == 0, &this_batch.batch),
+              &this_batch.signature
+            ));
 
             batch = Some(this_batch.clone());
           }