mcs: fix gin race by using a global once

[CabinfeverB]

What problem does this PR solve?

Issue Number: Close #7484

What is changed and how does it work?

as title

Check List

Tests

• Unit test
• Integration test

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lhy1024
• rleungx

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[codecov]

Codecov Report

Merging #7778 (4702021) into master (cd0ffba) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7778      +/-   ##
==========================================
- Coverage   73.69%   73.66%   -0.03%     
==========================================
  Files         432      432              
  Lines       47805    47793      -12     
==========================================
- Hits        35229    35206      -23     
- Misses       9572     9582      +10     
- Partials     3004     3005       +1     

Flag	Coverage Δ
unittests	73.66% <100.00%> (-0.03%)	⬇️

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[CabinfeverB]

/merge

[ti-chi-bot]

@CabinfeverB: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

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

Commit hash: 4702021