v20.05: gnome software: skip first run dialog

This dialog explains, how GNOME software is a front-end for multiple app formats. This is not useful in postmarketOS, the primary source for apps are the apk repositories. Also the apps are free of charge, "Let's go shopping" (the big close button of the dialog) sounds a bit weird in this context.

Disable the dialog for a smoother experience: we just want people to open it when they run postmarketOS for the first time and check for updates.

Note for when this gets upstreamed into GNOME software, and we don't carry our own version in postmarketOS, but use Alpine's version instead: then we should be able to set the dconf key before starting GNOME software the first time.

I've verified that this patch works.