Fix checkout in helm-publish workflow (#704)

*Issue #, if available:* The second checkout was overwriting the working
directory and removing the `charts/` folder from the first checkout.

*Description of changes:* Added path: main-branch to check it out to a
subdirectory instead.


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Signed-off-by: Renan Magagnin <renanmag@amazon.co.uk>

Author: renanmagagnin

.github/workflows/helm-publish.yaml

@@ -25,6 +25,7 @@ jobs:
         uses: actions/checkout@v6
         with:
           ref: main
+          path: main-branch
           sparse-checkout: |
             scripts/verify-helm-images.sh
           sparse-checkout-cone-mode: false
@@ -41,7 +42,7 @@ jobs:
           sudo chmod +x /usr/local/bin/crane
           crane version
       - name: Verify all images exist before publishing
-        run: ./scripts/verify-helm-images.sh
+        run: ./main-branch/scripts/verify-helm-images.sh
       # TODO: Uncomment after verifying the workflow works correctly
       # - name: Run chart-releaser
       #   uses: helm/chart-releaser-action@v1.7.0