fix(ci): rehearse a fixture release once per push

[pdobacz]

Minor fix to our CI job which rehearses building fixture releases: Previously, a push to a branch with an open pull request fired both triggers, and the two events land in different concurrency groups, so neither run cancelled the other. PR fixes this

Confidence Score: 5/5

The PR appears safe to merge.

Summary

Removes the pull_request trigger so a push to from-upstream or eips/** no longer starts two fixture rehearsals.

• Covered branch pushes start one fixture-release rehearsal, even when a pull request is open.

_{Reviews (1) · Last reviewed commit: "fix(ci): rehearse a fixture release once..." · Reviewed by Greptile}