chore(tools): update lambda runtime tests github action (#32788)

The GH Action workflow was missing the required permission to update the PR to automatically update the integ test cases

### 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)

----

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

.github/workflows/lambda-runtime-tests.yml

@@ -8,6 +8,8 @@ jobs:
   update-lambda-tests:
     if: github.repository == 'aws/aws-cdk'
     runs-on: ubuntu-latest
+    permissions:
+      contents: write
     steps:
       - uses: actions/checkout@v4
         with:
@@ -44,4 +46,6 @@ jobs:
           git config --global user.email '[email protected]'
           git add .
           git commit -m "chore: update lambda runtime integration tests"
-          git push origin ${{ github.event.pull_request.head.ref }}
+          git push origin ${{ github.event.pull_request.head.ref }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}