Add step to generate Flutter plugin registrants

Added step to generate Flutter plugin registrants during the build process.

Author: dddevid

.github/workflows/build-ios.yml

@@ -39,6 +39,9 @@ jobs:
       - name: Install dependencies
         run: flutter pub get
 
+      - name: Generate Flutter plugin registrants
+        run: flutter pub get && flutter build ios --config-only
+
       - name: Setup Xcode
         uses: maxim-lobanov/setup-xcode@v1
         with: