Run release testing on PR, not push (#5718)
## Summary
This job runs whenever I put up a PR to bump the version, which is
really useful. But then it also runs again when I merge, and then _that_
job tends to get cancelled immediately, because I run the _actual_
release job, which triggers the cancel-concurrent-runs flow. (See, e.g.,
https://github.com/astral-sh/ruff/actions/runs/5534191373.)
I think it makes sense to run these on PR (when editing `pyproject.toml`
and friends), but not again on merge.
