appimage: Use PREFIX/lib as libdir for all packages that are built

Many packages already used PREFIX/lib while most cmake-using packages
(except Qt) used PREFIX/lib64 as libdir. The AppImage deployment tool
puts all libraries in AppDir/usr/lib so that it makes sense that we also
use PREFIX/lib as install directory for the packages that we built.

• GnuPG-bug-id: T7509