fix(flutter): correct iOS artifact name to include run number

BrowserStack iOS job was trying to download 'flutter-ios-ipa' but
the Build iOS job uploads 'flutter-ios-ipa-${{ github.run_number }}'.

This was causing the error:
Unable to download artifact(s): Artifact not found for name: flutter-ios-ipa

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: Teodor Ciuraru

.github/workflows/flutter-ci.yml

@@ -675,7 +675,7 @@ jobs:
       - name: Download iOS IPA
         uses: actions/download-artifact@v4
         with:
-          name: flutter-ios-ipa
+          name: flutter-ios-ipa-${{ github.run_number }}
           path: flutter_app/build/ios/ipa/
 
       - name: Setup Flutter