cerbero: Force -Werror for gstreamer recipes on CI

We don't want this enabled by default in Cerbero because it causes unnecessary build regressions in the wild for users. However, we should enable it on the CI so we can catch warnings. Depends on: gstreamer!86 (merged), gst-plugins-base!115 (merged), gst-plugins-good!107 (merged), gst-plugins-bad!182 (merged), gst-editing-services!47 (merged)