[infra] GitHub actions no longer completes a build or test

### Context

As of a couple days ago, all GitHub actions now fail on any new PR on this repo.

The site still deploys, and preview URLs still work, so this does not block content changes.
However, it increases the risk of infrastructure changes.

Augustine did some investigating as a possible starting point (rephrased from chat):

```
There is a `BlockingRender` that exists (which makes some async work sync). It uses `Atomic.wait`, and `Atomics.notify`. At some point it may be that there are so many things happening, that there are not enough cycles for the worker thread to pick up the render request. They hang until one times out.
```

Start with the failing GitHub Actions runs and investigate the BlockingRender path described in the issue, including its use of Atomics.wait and Atomics.notify. Determine why new pull requests hang or time out, then verify that builds and tests complete successfully in GitHub Actions.

Written by the indexing model from the issue text.