Display build commit status in APG PR contributions

[howard-e]

Fix #391

Testing this work can be done by:

1. Making personal fork of w3c/aria-practices
2. Making personal fork of w3c/wai-aria-practices
3. Create a Personal Access Token (PAT) with repo and workflow permissions. Include that PAT as W3CGRUNTBOT_TOKEN actions secrets in both personal forks' settings
4. Targeting your aria-practices fork in your wai-aria-practices fork's .gitmodules
5. Ensuring the 'Trigger PR update for WAI-APG site' action is enabled on your aria-practices fork
6. Ensuring the 'Create or update branch and pull request from APG trigger' action is enabled on your wai-aria-practices fork
7. Making the same change here for scripts/pr-create/index.js on your wai-aria-practices fork default branch and observing how build commit statuses are shared back to your aria-practices fork's PRs. (NB. Forcing a failure to test that path could also be as easy as setting const isSiteFilesUpdateSuccess = false)

eg: howard-e/aria-practices#117

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