* Support running multiple instances without collision

This uses a combination of three Github-set env vars that, together, are
guaranteed to be unique within the scope of a repo and all its
workflows:

- GITHUB_WORKFLOW: the name of the workflow (or file path, if no name is given)
- GITHUB_JOB: the ID of the job
- GITHUB_ACTION: the ID of the step (or a formatted name of the repo
  where the action resides, i.e.
  `__mheap_github-action-required-labels`)

Without this, running multiple instances of this action with comments
enabled would result in them clobbering each others' comments.

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Co-authored-by: Michael Heap <m@michaelheap.com>