find python via env

This allows these scripts to run through the active version of the
virtualenv python when invoked via tox.

Change-Id: Ib52f475b7b20c34d62cfd179a1341da1a08a8b5c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/253974
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

run_tests

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # -*- coding:utf-8 -*-
 # Copyright 2019 The Android Open Source Project
 #

setup.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # -*- coding:utf-8 -*-
 # Copyright 2019 The Android Open Source Project
 #