Isolation read don't take effort after reboot tidb-server

[lzmhhh123]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Set the value of tidb-isolation-read-engines as ["tikv"] in tidb toml, but the sql plab choose the tiflash.

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

The sql plan for isolation read tikv is tikv.

3. Affected version (Required)

mysql> select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v4.0.0-rc-141-g7267747ae
Git Commit Hash: 7267747
Git Branch: jira-4.0
UTC Build Time: 2020-04-28 08:28:57
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.00 sec)

4. Root Cause Analysis

SysVars and session.variables aren't consistent in isolation-read-engines.