criu-ns: Update shebang line to python

CentOS 7 CI environment uses Python 2. To execute criu-ns
script in CentOS 7 changing the current shebang line to
python is required.

This reverse the changes made in a15a63fce0

Signed-off-by: Dhanuka Warusadura <csx@tuta.io>

scripts/criu-ns

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 import ctypes
 import ctypes.util
 import errno