Use official environment attribute of screenshots
requested to merge work/kloecker/microsoft-store-use-official-environment-screenshot-attribute into master
The upcoming Appstream 1.0 supports an "environment" attribute for marking screenshots for specific environments. Use screenshots marked with environment="windows" (instead of our custom os="windows") for the Microsoft Store. (We don't have to wait for Appstream 1.0 because older appstream-cli don't complain about it, so that we can already set the attribute in our AppData files.)
See ximion/appstream#530