zfs-load-key.sh: ${ZFS} is not the zfs binary

A change[1] was merged yesterday that should refer
to the zfs binary in the initramfs, but is actually
an unset shell variable.

This commit changes this line to call zfs directly
like the surrounding code.

[1]: cb5b875b273235a4a3ed28e16f416d5bb8865166

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Garrett Fields <ghfields@gmail.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Ben Cordero <bencord0@condi.me>
Closes #9780