workflow/labels: switch to a scheduled trigger

Instead of relying on workflow_run events, we now trigger the labeling
workflow by schedule. This avoids all permission/secrets problems of
running in the pull_request_review context - and also gets rid of the
"waiting for approval to run workflows" button for new contributors that
sometimes comes up right now.

Also, it's more efficient. Previously, the labeling workflow would run
on *any* pull_request_review event, which happens for all comments, too.
So quite a few runs.

This will cause a delay of up to 1 hour with the current settings until
approval labels are set. Depending on how long the job normally runs we
can adjust the frequency. The workflow is written in a way that will
work no matter what the frequency ends up being, even when it's
interrupted by transient GHA failures: It will always look at all PRs
which were updated since the last time the workflow ran successfully.

We also add the ability to run it manually via UI. This is useful:
- When fixing bugs in the labeler workflow to run it all the way back to
where the bug was introduced.
- When introducing new labels, to get a head start for a reasonable
amount of PRs immediately.

Of course, the workflow is also still run directly after Eval itself.
This is also the only case that the actions/labeler steps will run,
since they depend on the `pull_request` context.

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