DATA RACE in the (*MemDB).Dirty

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
==================
WARNING: DATA RACE
Read at 0x00c005c54b21 by goroutine 305800:
github.com/tikv/client-go/v2/internal/unionstore.(*MemDB).Dirty()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb.go:311 +0x97
github.com/tikv/client-go/v2/txnkv/transaction.(*KVTxn).IsReadOnly()
external/com_github_tikv_client_go_v2/txnkv/transaction/txn.go:1251 +0x52
github.com/pingcap/tidb/store/driver/txn.(*tikvTxn).IsReadOnly()
:1 +0x29
github.com/pingcap/tidb/session.(*LazyTxn).IsReadOnly()
:1 +0x4c
github.com/pingcap/tidb/executor.(*PointGetExecutor).get()
executor/point_get.go:471 +0xea
github.com/pingcap/tidb/executor.(*PointGetExecutor).getAndLock()
executor/point_get.go:369 +0x1ba
github.com/pingcap/tidb/executor.(*PointGetExecutor).Next()
executor/point_get.go:321 +0xf88
github.com/pingcap/tidb/executor.Next()
executor/executor.go:328 +0x5c3
github.com/pingcap/tidb/executor.(*SelectLockExec).Next()
executor/executor.go:1164 +0xeb
github.com/pingcap/tidb/executor.Next()
executor/executor.go:328 +0x5c3
github.com/pingcap/tidb/executor.(*UnionExec).resultPuller()
executor/executor.go:1861 +0x8e9
github.com/pingcap/tidb/executor.(*UnionExec).initialize.func1()
executor/executor.go:1814 +0x64
Previous write at 0x00c005c54b21 by goroutine 305799:
github.com/tikv/client-go/v2/internal/unionstore.(*MemDB).set()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb.go:348 +0x326
github.com/tikv/client-go/v2/internal/unionstore.(*MemDB).UpdateFlags()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb.go:250 +0x213a
github.com/tikv/client-go/v2/txnkv/transaction.(*KVTxn).LockKeys()
external/com_github_tikv_client_go_v2/txnkv/transaction/txn.go:1110 +0x20a5
github.com/pingcap/tidb/store/driver/txn.(*tikvTxn).LockKeys()
store/driver/txn/txn_driver.go:75 +0x105
github.com/pingcap/tidb/session.(*LazyTxn).LockKeys()
session/txn.go:442 +0x22e
github.com/pingcap/tidb/executor.doLockKeys()
executor/executor.go:1283 +0x30d
github.com/pingcap/tidb/executor.(*SelectLockExec).Next()
executor/executor.go:1215 +0x78f
github.com/pingcap/tidb/executor.Next()
executor/executor.go:328 +0x5c3
github.com/pingcap/tidb/executor.(*UnionExec).resultPuller()
executor/executor.go:1861 +0x8e9
github.com/pingcap/tidb/executor.(*UnionExec).initialize.func1()
executor/executor.go:1814 +0x64
Goroutine 305800 (running) created at:
github.com/pingcap/tidb/executor.(*UnionExec).initialize()
executor/executor.go:1814 +0x4d7
github.com/pingcap/tidb/executor.(*UnionExec).Next()
executor/executor.go:1887 +0xca
github.com/pingcap/tidb/executor.Next()
executor/executor.go:328 +0x5c3
github.com/pingcap/tidb/executor.(*HashAggExec).fetchChildData()
executor/aggregate.go:817 +0x325
github.com/pingcap/tidb/executor.(*HashAggExec).prepare4ParallelExec.func3()
executor/aggregate.go:854 +0x64
Goroutine 305799 (finished) created at:
github.com/pingcap/tidb/executor.(*UnionExec).initialize()
executor/executor.go:1814 +0x4d7
github.com/pingcap/tidb/executor.(*UnionExec).Next()
executor/executor.go:1887 +0xca
github.com/pingcap/tidb/executor.Next()
executor/executor.go:328 +0x5c3
github.com/pingcap/tidb/executor.(*HashAggExec).fetchChildData()
executor/aggregate.go:817 +0x325
github.com/pingcap/tidb/executor.(*HashAggExec).prepare4ParallelExec.func3()
executor/aggregate.go:854 +0x64
==================
```

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

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

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