Add /usr/bin/env to COPY_EXEC_LIST initramfs hook

5dc1ff29 changed the user space program to mount a zfs snapshot
from /bin/sh to /usr/bin/env. If the executable is not present
in the initramfs then snapshots cannot be automounted.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: James Dingwall <james.dingwall@zynstra.com>
Closes #5360
Closes #6913