chore(ci): use SHA as reference for integration test workflow instead…

… of branch. (#29787)

### 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
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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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*

.github/workflows/request-cli-integ-test.yml

@@ -38,7 +38,7 @@ jobs:
         with:
           # Needs to run with PROJEN_GITHUB_TOKEN because we need permissions to force push the branch
           token: ${{ secrets.PROJEN_GITHUB_TOKEN }}
-          ref: ${{ github.event.pull_request.head.ref }}
+          ref: ${{ github.event.pull_request.head.sha }}
           repository: ${{ github.event.pull_request.head.repo.full_name }}
       - name: Submit to test-pipeline
         run: |