Incorrect unix_timestamp result when utc time can be mapped to multi time in one zone

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```sql
set time_zone="Europe/Vilnius";
select unix_timestamp("2020-10-25 3:45");
```

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

```
mysql> select unix_timestamp("2020-10-25 3:45");
+-----------------------------------+
| unix_timestamp("2020-10-25 3:45") |
+-----------------------------------+
| 1603586700 |
+-----------------------------------+
```

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

```
mysql> select unix_timestamp("2020-10-25 3:45");
+-----------------------------------+
| unix_timestamp("2020-10-25 3:45") |
+-----------------------------------+
| 1603590300 |
+-----------------------------------+
```

Returned time should be another mapped time.

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

nightly(fc8bdb54c60)