ci: publish: get needed files from source

The script and template files are not part of the packaged core, so they
must be fetched from the source tree before being used.

Author: pillo79

Diff for: .github/workflows/package_core.yml

@@ -181,6 +181,12 @@ jobs:
       - name: Upload artifact
         run: aws s3 cp ${{ env.CORE_ARTIFACT }}.tar.bz2 s3://${{ secrets.S3_BUCKET }}/
 
+      - uses: actions/checkout@v4
+        with:
+          sparse-checkout: |
+            extra/gen_package_index_json.sh
+            extra/zephyr-core-template.json
+
       - name: Prepare package index snippet
         run: ./extra/gen_package_index_json.sh