Setting readonly=on when building a zrawdisk causes failures

[davidchisnall]

Running poudriere image -t zrawdisk, I get a failure because the zfs command emits this error:

cannot mount 'poudriereimageroot/var/empty': Invalid argument
property may be set but unable to remount filesystem

The script then exits because this command failed, but the failure is benign (we don't need this mounted on the host).

This is on 14-CURRENT/AArch64.