Since "flatpak-builder" action uses deprecated "upload-artifact"
version, that can stop working soon, disable it and upload ourselves.
This is supported in master, but not in any tagged version,
so for now switch to using action from master branch.
Also bump used runtime to GNOME 47 when building.
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).
