ZTS: fix zfs_copies_006_pos test on Ubuntu 20.04 (#16409)

This test was failing before:

• FAIL cli_root/zfs_copies/zfs_copies_006_pos (expected PASS)

Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>