18.04 is out of security support, and cmake for 18.04 is no longer
available via kitware's debian repository (or at least fetching it
failed for one build).
Also remove linearization from qpdf-manual.pdf. It's a small file, and
removing the dependency on the qpdf executable significantly shortens
build times.
The official, signed AppImage will be build locally for now for
security of my key, but I still want it to be built in a clean,
controlled environment based off a suitably old Linux distribution for
maximal portability in the AppImage.
