[release-0.21] fix iam kubeconfig generation in workload clusters

[eks-distro-pr-bot]

This is an automated cherry-pick of #9048

/assign tatlat

[eks-distro-bot]

Hi @eks-distro-pr-bot. Thanks for your PR.

I'm waiting for a aws member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ahreehong]

/approve
/lgtm

[eks-distro-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahreehong

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[tatlat]

/ok-to-test

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Please upload report for BASE (release-0.21@d5c53a1). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/clustermanager/cluster_manager.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##             release-0.21    #9049   +/-   ##
===============================================
  Coverage                ?   72.36%           
===============================================
  Files                   ?      578           
  Lines                   ?    45416           
  Branches                ?        0           
===============================================
  Hits                    ?    32866           
  Misses                  ?    10820           
  Partials                ?     1730           

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