Inconsistent stale read check result with prepared `START TRANSACTION`

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```sql
PREPARE stmt1 FROM 'START TRANSACTION READ ONLY AS OF TIMESTAMP NOW() - INTERVAL 5 SECOND';
EXECUTE stmt1;
EXECUTE stmt1;
```

### 2. What did you expect to see? (Required)

If not using prepared statement, the second start succeeds:

```sql
START TRANSACTION READ ONLY AS OF TIMESTAMP NOW() - INTERVAL 5 SECOND;
START TRANSACTION READ ONLY AS OF TIMESTAMP NOW() - INTERVAL 5 SECOND;
```

So, I expect it to also succeed when using prepared statement.

### 3. What did you see instead (Required)

The second `EXECUTE` raises `ERROR 1105 (HY000): only support read-only statement during read-only staleness transactions`.

### 4. What is your TiDB version? (Required)

6.5