workflows/labels: run on every PR eventually

This replaces the manual dispatch trigger with a batched run through all
pull requests every day. This has the small benefit of not having to
worry about backfilling labeling after fixing bugs - and the much bigger
one in being able to handle merge-conflict and stale labels properly
later. For those, it's inevitable to eventually scan through all PRs.

At this stage, the vast majority of PRs will still be skipped, because
there won't be an eval run with artifact available. This will be
improved in the next step.

Technically, the workflow_dispatch trigger is kept to allow easily
testing this in forks, where the scheduled jobs are disabled. The
triggered job will behave similar to the scheduled job, though, and have
no special inputs.

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