tools: use '/usr/bin/env python' instead of '/usr/bin/python'

* This is how it is done in other scripts in the AOSP tree, as
  '/usr/bin/python' may be python3 on some distros, which isn't
  supported yet.

Change-Id: I0d9857fc6a122f505953fddcd6244dad75b1838a
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_art into cm-11.0