ci: only report build status to monitoring on stable branch

Feature/version branches (e.g. the Fedora 44 bump) send the healthchecks
"start" ping but never the success ping when their build fails, which
makes monitoring.lablans.net raise false alarms. Gate both monitoring
pings on github.ref_name == 'stable' so only stable-branch builds report.

Author: claude

.github/workflows/build.yml

@@ -86,6 +86,7 @@ jobs:
       UPSTREAM_IMAGE: ${{ needs.wait-for-new-upstream-image.outputs.upstream_image }}
     steps:
       - name: Report to monitoring (start)
+        if: github.ref_name == 'stable'
         shell: bash
         env:
           URL: ${{ secrets.MONITORING_PUSH_URL }}
@@ -162,6 +163,7 @@ jobs:
           key: upstream-digest-${{ needs.wait-for-new-upstream-image.outputs.upstream_digest }}
 
       - name: Report to monitoring (success)
+        if: github.ref_name == 'stable'
         shell: bash
         env:
           URL: ${{ secrets.MONITORING_PUSH_URL }}