Have a 'post' entrypoint to print a summary of what happened

[ChrisCarini]

See docs for details: https://docs.github.com/en/actions/creating-actions/metadata-syntax-for-github-actions#runspost-entrypoint

Right now, $post_loop_messages doesn't get printed 'post-loop', as we exit <code> right away. 'post' entrypoint could help address this.