Do not update buildout.coredev on PRs that have a [ci-skip] message

[gforcada]

As @mauritsvanrees found out, we are only partially following the [ci-skip] message when merging a PR.

Either we check for [ci-skip] before firing the MergedPullRequest event (on these lines) or we rather do it on the event subscriber (on these class).

The second feels more natural, as there are plenty of checks there already.

We might even recycle some of the logic on the direct commits case ♻️

🍀

[gforcada]

Finally, after a few tries and back and forth, plone/plone.registry#28 was merged with a [ci-skip] message on the PR commit message and mr.roboto did not update buildout.coredev 🎉