gitpod PR check: The check finishes long before the prebuild is complete

Bug description

If you have a project set up with

    pullRequests: true
    pullRequestsFromForks: true
    addCheck: true

then a PR will include a check for gitpod prebuilding the workspace. And it works.

The problem here is that the check goes green long before the prebuild is complete, sometimes several minutes before.

Steps to reproduce

1. Open a PR on https://github.com/rfay/test-gitpod-prebuild - just use the GitHub editor to make a small change to the README.
2. Wait for the PR's gitpod check to go green
3. Click the link on the check. You'll see it still prebuilding and far from done.

Expected behavior

The completion of the gitpod check should indicate that the prebuild is complete.

Example repository

https://github.com/rfay/test-gitpod-prebuild but it's just a playground to demonstrate this. Feel free to create PRs there

Anything else?

Thanks for a great project that gets better all the time.

Start by reproducing the behavior with the pullRequests, pullRequestsFromForks, and addCheck settings in the linked test-gitpod-prebuild repository, then inspect the Gitpod PR check and prebuild status it links to. Done means the check stays pending until the prebuild completes and turns green only afterward.