use the workflow cpp sdk version variable

Author: AlmostMatt

.github/workflows/build_starter.yml

@@ -104,7 +104,7 @@ jobs:
             echo "::set-output name=apis::'analytics,auth,crashlytics,database,dynamic_links,firestore,functions,installations,messaging,remote_config,storage'"
             echo "::set-output name=unity_version::2019"
             echo "::set-output name=should_trigger_package::true"
-            echo "::set-output name=firebase_cpp_sdk_version::"
+            echo "::set-output name=firebase_cpp_sdk_version::${{ github.event.inputs.firebase_cpp_sdk_version }}"
             echo "::set-output name=unity_branch::"
             echo "::set-output name=additional_cmake_flags::"
             if [[ "${{ github.event_name }}" == "schedule" ]]; then