flatpak, CI, nightly: Use the 3.32 SDK/Runtime
In a recent runtime update that intended to move Mesa to an runtime extension, the freedesktop-sdk runtime maintainers have not managed to expose libgbm into the SDK. virglrenderer depends on it to build.
See https://gitlab.com/freedesktop-sdk/freedesktop-sdk/issues/822
This commit can be reverted once https://gitlab.com/freedesktop-sdk/freedesktop-sdk/merge_requests/1553 gets into the flatpak repository.