Run smoke test with appimagetool during CI pipelines

[TheAssassin]

To make sure that appimagetool can work with the produced binaries, we should build it and run a short smoke test (i.e., build an AppImage, e.g., of appimagetool itself). That way, we make sure we don't accidentally break the relation between the two projects.

[probonopd]

Won't that increase the build time in this repository tremendously? What is the advantage over just telling the appimagetool repository to trigger a build? (You referred to "dependency hell" but I don't quite understand how the solution proposed above is better.)