dev: Check the Heroku build status during our GitHub Actions deploy job

[tsibley]

We'd been assuming builds on Heroku were always successful, which meant the GitHub Actions status could (and did) mask build issues on Heroku.

Resolves https://github.com/nextstrain/nextstrain.org/issues/726.

Checklist

• [x] Tested the snippets manually; testing the workflow is not possible without merge first
• [x] Checks pass

[tsibley]

Worked in the success condition: https://github.com/nextstrain/nextstrain.org/actions/runs/6486469947/job/17614896676#step:6:1

We'll see I guess in the future if it works in the failure condition. I don't think we need to intentionally fail a deploy to test it (but we could).