ZTS: Fix 'could not unmount datasets' on Alma 9 (#15542)

Many tests are failing on AlmaLinux 9 because ZTS could not destroy the
pool in cleanup. This was due to $PWD being set to '.' instead of the
expected full path. This patch sets $PWD to the full path.

Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Don Brady <don.brady@delphix.com>