kv: fix data race in TestReplicaTxnIdempotency

Fixes cockroachdb#107435.

This commit deflakes `TestReplicaTxnIdempotency`. The race seen in the
test was between an asynchronous read of a key attached to a raft
proposal during a timestamp cache update and a mutation of that same
underlying byte buffer by the test harness. We now make sure to clone
the key before mutating the buffer.

Release note: None

Author: nvb

pkg/kv/kvserver/replica_test.go

@@ -3728,12 +3728,12 @@ func TestReplicaTxnIdempotency(t *testing.T) {
 				return runWithTxn(nil, &args)
 			},
 			afterTxnStart: func(txn *roachpb.Transaction, key []byte) error {
-				args := deleteRangeArgs(key, append(key, 0))
+				args := deleteRangeArgs(key, roachpb.Key(key).Clone().Next())
 				args.Sequence = 2
 				return runWithTxn(txn, &args)
 			},
 			run: func(txn *roachpb.Transaction, key []byte) error {
-				args := deleteRangeArgs(key, append(key, 0))
+				args := deleteRangeArgs(key, roachpb.Key(key).Clone().Next())
 				args.Sequence = 2
 				return runWithTxn(txn, &args)
 			},