AppImage build fails

[Insprill]

Describe the bug
The AppImage build has been failing as of late. It's been temporarily disabled for now, but the last failure can be seen here.

@swiesmann would you be able to look into this?

[goolic]

I took a look at this and the pkg2appimage folks seem to require ubuntu 18.04 builders to garantee that the deb package is compatible.

AppImageCommunity/pkg2appimage#530

Maybe i'll look into a workaround? off the top of my head we can try to force dpkg-deb -b to use older compression, or we can try to recompress the package before running pkg2appimage