Re-opened PRs can potentially merge requirements

[klutchell]

Discussion: https://balena.zulipchat.com/#narrow/stream/348930-balena-io.2Fflowzone/topic/Flowzone.20skipped.20almost.20everything/near/455074162

Here is the suspected order of events where a problem may occur:

1. PR is closed manually for any number of reasons (unmerged)
2. PR close event triggers the workflow and runs the Custom Clean job, and maybe some others
3. At some point in the future, the PR is reopened manually or by Renovate
4. Flowzone workflows are not currently triggered by reopen event types so nothing happens
5. The last successful workflow run at this point was on the previous close event, so the All jobs required step has a result of skipped
6. Jobs with a skipped result are unusually considered to be passing in branch protections, so at this point the PR can be merged manually by clicking Merge

The possible solutions vary:

• Run All jobs on all event types, but mark as failed if the PR is not open
• Run workflow on reopen events (this requires changing the calling workflows in 100s of repositories)
• ???

The core of this issue seems to be that Github PR triggers do not include closed (including merged) events by default, and they must be added. Combine this with the fact that Github treats skipped jobs as meeting merge requirements puts us in this situation.