Consider to bundle the runtime

The downloading mechanism is not robust over time:

- The runtime repo name might change
- The runtime filename might change

Errors caused by this should happen at build time, when developers can fix it, rather than at run time, when users are affected.

Hence we should consider to bundle the runtime in the appimagetool AppImage.

But the bare minimum needed, I think, is that changes in type2-runtime need to trigger a build of appimagetool. So that any potential issues are seen by developers immediately.