session: fix set session variable make tidb_snapshot unwork (#28677)

[ti-srebot]

cherry-pick #28677 to release-5.2
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/28683

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/28683:release-5.2-417c7358ef31

---

Signed-off-by: yisaer disxiaofei@163.com

What problem does this PR solve?

tidb_snapshot won't work after setting any session variable

What is changed and how it works?

fix set session variable make tidb_snapshot unwork

Check List

Tests

• Unit test

Release note

Fix the bug that setting any session variable will make `tidb_snapshot` unwork.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• xhebox
• zimulala

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[ti-srebot]

/run-all-tests

[ti-srebot]

@Yisaer you're already a collaborator in bot's repo.

[zimulala]

Please fix conflicts @Yisaer

[zimulala]

PTAL @xhebox @nolouch

[zimulala]

LGTM

[zhouqiang-cl]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 25f7834

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.