txn: unexpected select executor under update

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```
create table t(c1 int key, c2 int, c3 int, unique key uk(c2));

insert into t values(1, 2, 3);

begin;
explain update t set c2 = c2 where c3 = 3;
```

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

```
+-----------------------------+----------+-----------+---------------+--------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+----------+-----------+---------------+--------------------------------+
| Update_5 | N/A | root | | N/A |
| └─TableReader_10 | 10.00 | root | | data:Selection_9 |
| └─Selection_9 | 10.00 | cop[tikv] | | eq(test.t.c3, 3) |
| └─TableFullScan_8 | 10000.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+-----------------------------+----------+-----------+---------------+--------------------------------+
5 rows in set (0.00 sec)

```

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

```
+-----------------------------+----------+-----------+---------------+--------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+----------+-----------+---------------+--------------------------------+
| Update_5 | N/A | root | | N/A |
| └─SelectLock_7 | 10.00 | root | | for update 0 |
| └─TableReader_10 | 10.00 | root | | data:Selection_9 |
| └─Selection_9 | 10.00 | cop[tikv] | | eq(test.t.c3, 3) |
| └─TableFullScan_8 | 10000.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+-----------------------------+----------+-----------+---------------+--------------------------------+

```

### 4. What is your TiDB version? (Required)
```
select tidb_version();
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v6.2.0-alpha
Edition: Community
Git Commit Hash: 97f66c3fecebc89fa7e1472c4a53d7a3462432cf
```