The move to Clang in 85ed2847 didn't take into account caching,
Now, this is fixed or, better saying, reintroduced(?): e545116b
Also, flatpak-builder output was confined to the .log file to make
possible checking the end of the CI output, which was being cut.
AppImage is pretty fast to make, like the win crossbuild; and portable,
being very appropriate to do quick tests on Linux when pushing to git.
The overall organization of Debian jobs was changed to take advantage
of this and make things less complicated (but less clear at first sight).
I reinforce that this was the most efficent way to make the AppImage.
