ci: Fix secrets for version index workflow (#7906)

See also PR #7892 and #7900.

Author: joeyparrish

.github/workflows/demo-version-index.yaml

@@ -7,6 +7,9 @@ on:
     # Allows calling this from another workflow with "uses:".
     # The release workflow will trigger this as a final step after updating the
     # appspot deployment.
+    secrets:
+      APPENGINE_DEPLOY_KEY:
+        required: true
   push:
     branches:
       - main

.github/workflows/release-please.yaml

@@ -207,3 +207,5 @@ jobs:
     name: Deploy Demo Version Index
     needs: [appspot]
     uses: ./.github/workflows/demo-version-index.yaml
+    secrets:
+      APPENGINE_DEPLOY_KEY: '${{ secrets.APPENGINE_DEPLOY_KEY }}'