Authenticate automated commits as Hypothesis GitHub App

Automated PRs from the [`update_repos.yml` workflow](https://github.com/hypothesis/cookiecutters/blob/main/.github/workflows/update_repos.yml) authenticate themselves to GitHub as [GitHub Hypothesis App](https://github.com/apps/hypothesis-github-app) but the author of the individual commits is set to **GitHub Actions Cookiecutter Workflow** ([example PR](https://github.com/hypothesis/h-periodic/pull/285), [example commit](https://github.com/hypothesis/h-periodic/pull/285/commits/2dd03232651010deff41f209afc16ff14c0fbce6)).

It would be nicer if the commits themselves were also recognised by GitHub as coming from Hypothesis GitHub App and I think this is possible. We need to set the git email address (`git config --global user.email "..."`) to the form `userID + name[bot] + @users.noreply.github.com` and apparently we can get the right email and user ID from the response from the endpoint (to a request authenticated as the app).