ZTS: Fix /usr/bin/env: 'python2': No such file or directory

Since 4f342e4 env(1) must be able to find a "python2" executable in
the "constrained path" on systems configured with --with-python=2.x
otherwise the ZFS Test Suite won't be able to use Python scripts.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #9325

tests/zfs-tests/include/commands.cfg

@@ -95,6 +95,7 @@ export SYSTEM_FILES='arp
     ps
     pwd
     python
+    python2
     python3
     quotaon
     readlink