Example workflow: Don't run the bot when PR is from a fork

[desrosj]

What?

This updates the example workflow file to prevent pull_request_review and pull_request_review_comment events from attempting to run the bot.

Why?

For PRs that originate from forked repositories, pull_request_review and pull_request_review_comment events do not have the permissions necessary to comment on the PR.

See #57.

[github-actions]

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Core SVN

Core Committers: Use this line as a base for the props when committing in SVN:

Props desrosj, jorbin.

GitHub Merge commits

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: desrosj <[email protected]>
Co-authored-by: aaronjorbin <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.