Update build workflow to use the C++ version input (#474)

* use the firebase_cpp_sdk_version input

Author: AlmostMatt

.github/workflows/build_starter.yml

@@ -95,6 +95,7 @@ jobs:
             echo "::set-output name=unity_version::${{ github.event.inputs.unity_version }}"
             echo "::set-output name=should_trigger_package::${{ github.event.inputs.should_trigger_package }}"
             echo "::set-output name=runIntegrationTests::${{ github.event.inputs.runIntegrationTests }}"
+            echo "::set-output name=firebase_cpp_sdk_version::${{ github.event.inputs.firebase_cpp_sdk_version }}"
             echo "::set-output name=unity_branch::${{ github.event.inputs.unity_branch }}"
             echo "::set-output name=additional_cmake_flags::${{ github.event.inputs.unity_branch }}"
           else