ci: only cancel pull_request_stats on PR updates (#90104)
This avoids cancelling on canary branch when a commit is merged while
previous commit is still running
## Summary
- keep cancel-in-progress behavior for pull_request runs
- stop canceling in-progress runs for follow-up pushes to canary by
using a per-run concurrency group for push events
x-ref:
https://github.com/vercel/next.js/actions/runs/22113161316/job/63914496141
