ZTS: Fix ksh path in btree tests

ZTS: Fix ksh path in btree tests

Every other ksh script has /bin/ksh in the shebang.
If we're going to assume a path, we can at least be consistent.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9845