appimage: Fix cross-distribution build compatibility
No one needs multiarch support inside a single-arch appimage, so hardcode the libdir to $PREFIX/lib instead of letting the build distribution randomly decide on lib64, lib/x86_64-linux-gnu etc.
No one needs multiarch support inside a single-arch appimage, so hardcode the libdir to $PREFIX/lib instead of letting the build distribution randomly decide on lib64, lib/x86_64-linux-gnu etc.