Demo index workflow runs too early

[joeyparrish]

The release trigger for the demo index workflow is "working" in the sense that it runs when a release is made, but it's not quite right.

The release workflow publishes the GitHub release, but then there is a lag for it to publish a new demo to appspot. Meanwhile, the demo index workflow is already running, where it pulls a list of appspot demo versions. So the demo index is updated before the new appspot version is out.

The nightly scheduled update to the demo index fixes it within 24 hours, but that's still not ideal.