br: Fix the query of variable tidb_cluster

[chenmin1992]

What problem does this PR solve?

Issue Number: close #39953

Problem Summary:

What is changed and how it works?

Check List

Tests

• [ ] Unit test
• [ ] Integration test
• [x] Manual test (add detailed scripts or steps below)
• [x] No code

Side effects

• [ ] Performance regression: Consumes more CPU
• [ ] Performance regression: Consumes more Memory
• [ ] Breaking backward compatibility

Documentation

• [ ] Affects user behaviors
• [ ] Contains syntax changes
• [ ] Contains variable changes
• [ ] Contains experimental features
• [ ] Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fix the query of variable tidb_cluster

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All committers have signed the CLA.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 73.3143%. Comparing base (2ea253e) to head (68c52d8). :warning: Report is 7861 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #39954        +/-   ##
================================================
- Coverage   73.3395%   73.3143%   -0.0253%     
================================================
  Files          1076       1080         +4     
  Lines        341769     344803      +3034     
================================================
+ Hits         250652     252790      +2138     
- Misses        74955      75618       +663     
- Partials      16162      16395       +233     

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