revisit prettier automation

[pkra]

In #2464 I had re-activated prettier after it had been disabled in #2307.

I don't know if the dry-run option was really relevant. But it disabled the automation and didn't surface the errors. I don't think that was what we wanted.

I'm not sure if the dry-run option was necessary. It looks to me that the failure should be caught by handle-commit-failure anyway?

If it was, then we could change the action to run prettier on commits to main (instead of on PRs).

Alternatively, we could give up on automated formatting and just use dry-run, but surfacing errors (which authors then need to fix).

Or there are other ideas.