Move the failed workflow re-running to a central workflow_run triggered workflow

[johnbillion]

Trac ticket: https://core.trac.wordpress.org/ticket/64298

[github-actions]

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[desrosj]

I think this looks good overall. However, I have a few concerns.

First, runs that trigger with workflow_run appear to happen from trunk and do not link back to the triggering workflow (see these workflow runs).

Though I do like having the Slack message represented as a job within the original workflow run, I don't feel strongly enough to insist it remains implemented as it is today. But it's a tradeoff worth considering. Perhaps we could add log output that notes the triggering workflow run (see Core-64299).

And second, I'm not certain, but this change will likely break for all old branches. Or, the workflow will run twice: once with the old workflow_dispatch approach (which should fail), and another time using the new workflow_run.