ZTS: Interpret env vars in faketty on FreeBSD

This was missed in review. On FreeBSD, script does not understand
environment variables being passed as a command.

Use env to make faketty handle env vars on FreeBSD.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #9981

tests/zfs-tests/include/commands.cfg

@@ -114,6 +114,7 @@ export SYSTEM_FILES_COMMON='arp
 export SYSTEM_FILES_FREEBSD='chflags
     compress
     dumpon
+    env
     fsck
     getextattr
     gpart