Repo must be cloned, not forked

[estensen]

This repo has to be cloned, not forked. If not you won't get a check_run event:

"The Checks API only looks for pushes in the repository where the check suite or check run were created. Pushes to a branch in a forked repository are not detected and return an empty pull_requests array and a null value for head_branch."
GitHub docs

[estensen]

check_run is actually not run for a cloned repo either

[estensen]

The instructions only work if a check_run event is fired. In my case I'm not running a third-party tool firing of a such event.
Created a workaround. I'll create an upstream PR if you're interested :)

merge_and_cleanup_branch.py

...
if __name__ == '__main__':
    github_token = os.environ["INPUT_GITHUB_TOKEN"]
    github_repository = os.environ["GITHUB_REPOSITORY"]

    github_event_path = os.environ["GITHUB_EVENT_PATH"]
    event_data = json.load(open(github_event_path))

    pull_request = event_data["pull_request"]

    sess = get_session(github_token)

    pr_data = sess.get(pull_request["url"]).json()
    pr_user = pr_data["user"]["login"]
    print(f"*** This PR was opened by {pr_user}")

    if pr_user != "estensen":
        print("*** This PR was opened by somebody who isn't me; requires manual merge")
        neutral_exit()

    pr_number = event_data["number"]
    pr_src = pull_request["head"]["ref"]
    pr_dst = pull_request["base"]["ref"]

    print(f"*** Checking pull request #{pr_number}: {pr_src} ~> {pr_dst}")

    pr_title = pr_data["title"]
    print(f"*** Title of PR is {pr_title!r}")
    if pr_title.startswith("[WIP] "):
        print("*** This is a WIP PR, will not merge")
        neutral_exit()

    print("*** This PR is ready to be merged.")
    merge_url = pull_request["url"] + "/merge"
    sess.put(merge_url)

    print("*** Cleaning up PR branch")
    pr_ref = pr_data["head"]["ref"]
    api_base_url = pr_data["base"]["repo"]["url"]
    ref_url = f"{api_base_url}/git/refs/heads/{pr_ref}"
    sess.delete(ref_url)