ci: compress squashfs with zstd

Compress squashfs with zstd since that now we have
CONFIG_SQUASHFS_ZSTD=y in all our guest kernels.

In my tests it is 78MB vs 85MB (an 8.2% reduction)

Signed-off-by: Pablo Barbáchano <pablob@amazon.com>

resources/rebuild.sh

@@ -85,10 +85,9 @@ EOF
  id_rsa=$OUTPUT_DIR/$ROOTFS_NAME.id_rsa
  sudo cp id_rsa $id_rsa
 
- # -comp zstd but guest kernel does not support
  rootfs_img="$OUTPUT_DIR/$ROOTFS_NAME.squashfs"
  sudo mv $rootfs/root/manifest $OUTPUT_DIR/$ROOTFS_NAME.manifest
- sudo mksquashfs $rootfs $rootfs_img -all-root -noappend
+ sudo mksquashfs $rootfs $rootfs_img -all-root -noappend -comp zstd
  rootfs_ext4=$OUTPUT_DIR/$ROOTFS_NAME.ext4
  dir2ext4img $rootfs $rootfs_ext4
  sudo rm -rf $rootfs