Flatpak: Switch to master branch of the runtime.

All the blocker bugs have been fixed but the gnome-sdk hasn't been rebuilt since then. Keeping this WIP till then.

Update the flatpak runtime so we use master again.

Given tha gnome-sdk//master is based on a new version of the Freedesktop-sdk the Rust-sdk extension can no longer be fetched from flathub, at least for now.

Instruction for how to get the new SdkExtension:

flatpak remote-add --user freedesktop https://cache.sdk.freedesktop.org/releases/
flatpak install freedesktop org.freedesktop.Sdk.Extension.rust-stable