chore(ci): use SHA as reference for integration test workflow instead of branch.

[AdnaneKhan]

Reason for this change

It is better to use an immutable commit reference for workflows that require approval, since the branch's head commit can change between the time of approval and workflow run execution.

Description of changes

Updated the ref parameter to use the SHA from the pull_request event instead of the ref.

Description of how you validated changes

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 8a73a00
• Result: SUCCEEDED
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