stop running link validator on every PR commit

[pjfanning]

The job is so brittle. We can afford to miss an occasional broken link.

We are really stressing repository.apache.org. #1585

See https://github.com/apache/pekko/actions/runs/12361018308

Can we just run it once a night?

[raboof]

The job is so brittle. We can afford to miss an occasional broken link.

We are really stressing repository.apache.org. #1585

(I'm not convinced it's us that's causing GitHub Actions runners to get blocked from repository.apache.org, but indeed we're impacted by it)

See https://github.com/apache/pekko/actions/runs/12361018308

Can we just run it once a night?

Yes, much in favour #1309

[pjfanning]

closing as duplicate of #1309