zstd decompression is a lot faster than xz (default). This is especially
noticeable when starting Firefox, Chromium or VS Code for the first
time; with zstd, the startup time is cut in half. Compression time is
also faster with zstd at the default level. The downside is that the
squashfs is larger by 138 MB. This is a tradeoff, but I think it's worth
it. Increasing the zstd compression level does not significantly reduce
the size and takes much longer to compress, so I left it at the default.
Before this, it loaded everything from the ISO into RAM, now it only
loads the squashfs. This saves about 110 MB of space in RAM, and reduces
boot time.
