Make appimagetool use zstd

[probonopd]

In anticipation of using https://github.com/AppImage/type2-runtime and

• Release versions/v1.0.md AppImageSpec#34

use zstd as the default compression in appiamgetool.

[probonopd]

Still getting

Generating squashfs...
/tmp/AppImageKit-build-8aSFXR/install-prefix/usr/bin/../lib/appimagekit/mksquashfs: Compressor "" is not supported!
/tmp/AppImageKit-build-8aSFXR/install-prefix/usr/bin/../lib/appimagekit/mksquashfs: Compressors available:
	gzip (default)
	xz

I guess I need the help of @TheAssassin to get CMake to build mksquashfs with zstd; most likely

https://github.com/AppImage/AppImageKit/blob/probonopd-zstd/cmake/dependencies.cmake

needs to be changed?

[TheAssassin]

Fixing this PR right now.

[probonopd]

Closing this PR since appimagetool will get its own repository.