ci(repo): skip sample app build when targeting master (#2720)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: xsahil03x

.github/workflows/stream_flutter_workflow.yml

@@ -167,7 +167,7 @@ jobs:
     name: build (${{ matrix.platform }})
     needs: gate
     runs-on: ${{ matrix.os }}
-    if: needs.gate.outputs.should_run == 'true'
+    if: needs.gate.outputs.should_run == 'true' && github.event_name == 'pull_request'
     timeout-minutes: 30
     strategy:
       fail-fast: false