ci: Add two flatpak jobs

ci: Add two flatpak jobs

When building GTK through the CI infrastructure, it would help to have
some ways of testing it; for instance, if we want to verify that theme
changes are useful, or if we want to run the result without necessarily
build it locally.

This is where flatpak comes in handy. By having the CI build a flatpak
buundle, and storing it as an artifact, of the GTK demos, we can easily
point developers and designers to an installable binary that won't break
their system, nor will require development tools and environments to
run.