use zstd for iso creation

[nrdxp]

As it stands now creating a DevOS ISO can take an extremely long time by virtue of the fact that nixpkgs upstream uses extremely hefty LZMA compression defaults for the squashfs image. The iso image module allows to change the compression arguments passed to squashfs and recent enough kernels support zstd compression for squashfs images.

Therefore, In order to be as user friendly as possible, and because relatively large thumb drives are fairly cheap nowadays, a default setting which uses zstd in place of lzma could save users a lot of time and frustration when making an installable DevOS iso image, while only sacrificing only a small bit of disk space.