Add a way to override to use fallback service file
Currently it's possible to override the service file by setting MAPS_SERVICE environment variable to point to a local file (and Maps will use that instead of downloading the normal service file).
This is useful for testing out new stuff.
Unfortunatly this is not really easy to do from a Flatpak container. As we already have a fallback service file that is used if for some reason the regular service file fails to download, we could add an environment variable to override falling back to the built-in service file direcltly, like:
FORCE_SERVICE_FALLBACK=true flatpak run org.gnome.Maps
This way, the developer can make changes directly in data/maps-service.json for testing out new things.