Self upgrade PRs don't run checks

[SgtCoDFish]

See e.g. #1545

The required check didn't run until I closed + reopened the PR. This PR should instead be automatically merged if tests pass. From a discussion with @inteon and @wallrj this is apparently a known issue.

At a minimum we should document this flaw. Ideally it should be fixed of course!

[SgtCoDFish]

@wallrj suggested the cause may be related to https://docs.github.com/en/actions/security-for-github-actions/security-guides/automatic-token-authentication#using-the-github_token-in-a-workflow - specifically:

When you use the repository's GITHUB_TOKEN to perform tasks, events triggered by the GITHUB_TOKEN, with the exception of workflow_dispatch and repository_dispatch, will not create a new workflow run. This prevents you from accidentally creating recursive workflow runs. For example, if a workflow run pushes code using the repository's GITHUB_TOKEN, a new workflow will not run even when the repository contains a workflow configured to run when push events occur.