The github actions Flatpak action only allows to specify single source
for installing missing dependencies. This makes impossible to build
some things from "gnome-nightly" while others from "flathub" repo.
Work around this limitation by manually installing missing SDK
extensions from Flathub prior to using this git action.
Rename flatpak-testing into flatpak-nightly so it reflects that it uses GNOME nightly sources now and builds itself every night (with nightly gstreamer too).
