use /usr/bin/env in shebang

This allows the path to the python binary to be overwritten using
environment variables (for example in virtualenvs).

make.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import sys
 import os

script-Z-preroll-generator.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 import xmlrpc.client
 import traceback
 import socket