Test beta release (27.2.0-beta4)

CI: Split Flatpak beta and stable branches

We currently publish the same build from the same branch
to Flathub. However, soon we'll need to build the Flatpak
manifest in different branches, and publish them in different
repositories.

Prepare for that by splitting the publish step in two: one
for Flathub, and another for Flathub Beta. Do that using
a matrix strategy.