[stubgen] Hermetic Python3 support

Test: lib/syscall/stubgen/stubgen.py --help
Bug: 197031257
Change-Id: Ia83eb0fbddab349b4608c02a972c0dbe6c12e84a
diff --git a/lib/syscall/stubgen/stubgen.py b/lib/syscall/stubgen/stubgen.py
index 2c9ac0f..bb057db 100755
--- a/lib/syscall/stubgen/stubgen.py
+++ b/lib/syscall/stubgen/stubgen.py
@@ -1,4 +1,6 @@
-#!/usr/bin/python
+#!/bin/sh
+"." "`dirname $0`/../../../../../trusty/vendor/google/aosp/scripts/envsetup.sh"
+"exec" "$PY3" "$0" "$@"
 
 # Copyright 2013-2017 Google Inc. +All rights reserved.
 #