install-tree.sh: use tar to capture symlinks properly

[ahesford]

Using find and install to copy files misses the convenience symlinks that we use in zfsbootmenu/bin. Even if we can identify the links with another find command, install will dereference them. The simplest solution is to just use tar, and --no-same-owner will ensure that ownership isn't taken from the repository but from the user actually doing the installation (although the permissions will still be taken from the repo). This seems to work as expected for an installation as a regular user and as root, and also appears to work with both GNU tar and bsdtar.