executor: make sure buildHashTable worker exits before IndexHashJoin finish (#31334)

[ti-srebot]

cherry-pick #31334 to release-5.0
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/31416

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

git push git@github.com:ti-srebot/tidb.git pr/31416:release-5.0-40c0567ab398

---

What problem does this PR solve?

Issue Number: close #31062

Problem Summary:

What is changed and how it works?

Before this commit, buildHashTable worker may be still running while IndexHashJoin has finished already.
This commit makes the Close function wait for buildHashTable worker to exit to fix the problem.

Check List

Tests

Test

Side effects

N/A

Documentation

N/A

Release note

None

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-srebot]

/run-all-tests

[ti-srebot]

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