Actually change the directory's mode. While it is enforced in the final
image via the METALOG, the directory under $NANO_WORLDDIR should also be
updated.
This is especially important when using $NANO_WORLDDIR as the mountpoint
for a null-mounted poudriere jail, since var/tmp must be world-writable
for poudriere bulk to succeed.
Fixes: 6eccd84bee75 ("nanobsd: Make tgt_dir2symlink() take a mode")