ci: skip multi-platform build-check on push, run only on PRs

The release workflow already builds wheels on all 4 platforms when a tag
is pushed. Restrict build-check to pull_request events to avoid duplicate
builds when pushing to master.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Kosthi

.github/workflows/ci.yml

@@ -62,6 +62,7 @@ jobs:
         run: pytest tests/ -x
 
   build-check:
+    if: github.event_name == 'pull_request'
     strategy:
       matrix:
         include: