checker: fix the conflict between tiflash learner and location labels

[JmPotato]

Cherry-pick for #6660.

What problem does this PR solve?

Issue Number: Close #6662.

What is changed and how does it work?

This PR only chooses the source store that is contained in the rule.

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• nolouch
• rleungx

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 🎉

Comparison is base (614ecda) 75.64% compared to head (77324b4) 75.69%.

Additional details and impacted files

@@                       Coverage Diff                       @@
##           release-6.5-20230620-v6.5.2    #6671      +/-   ##
===============================================================
+ Coverage                        75.64%   75.69%   +0.04%     
===============================================================
  Files                              330      330              
  Lines                            32982    32983       +1     
===============================================================
+ Hits                             24950    24966      +16     
+ Misses                            5878     5871       -7     
+ Partials                          2154     2146       -8     

Flag	Coverage Δ
unittests	75.69% <100.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/schedule/checker/rule_checker.go	81.58% <100.00%> (+0.06%)	⬆️

... and 24 files with indirect coverage changes

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