Switch to zstd for SquashFS compression in development ISO/USB images

refs #18655

This give us almost as good compression, but significantly faster compression.

For released images the picture is quite different and doing this switch would require more work (for details, see the GitLab issue).

This MR only impacts fastcomp, so only development images built on developers' machines are impacted, since we use defaultcomp on Jenkins and when building release images.