`gondolin build` fails on zfs filesystems with compression enabled for automatic rootfs size

[hyperparabolic]

getDirSizeKb under-reports directory sizes on a zfs filesystem with compression enabled.

du -sk reports the size of the files on the disk. With compression enabled this reports significantly less than what's required to copy those same files onto a ext4 filesystem, and you run out of space copying files to the rootfs.

Btrfs with compression likely sees the same results.

Adding the -A/--apparent-size flag to du should get the correct sizes.