fix(client-sts): disable stsClient cache

[cuyl]

sourceCreds can be expired so we can't cache the STSClient instance.

[AllanZhengYP]

Hi @cuyl Thank you for bringing this to attention. I will take a look at this.

[codecov-commenter]

Codecov Report

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@@           Coverage Diff           @@
##             main    #2268   +/-   ##
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  Coverage        ?   59.90%           
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  Files           ?      467           
  Lines           ?    24583           
  Branches        ?     5834           
=======================================
  Hits            ?    14726           
  Misses          ?     9857           
  Partials        ?        0           

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[aws-sdk-js-automation]

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• CodeBuild project: sdk-staging-test
• Commit ID: 36d3407
• Result: SUCCEEDED
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[AllanZhengYP]

Closing this PR as it has been replaced by #2353

[github-actions]

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