Use env to invoke python

/usr/bin/python may be python3. We should respect PATH to find the python
executable so it can be locally overridden to be python2.

Test: Build libc, repo upload
Change-Id: Iaddd7cd4a1c2177c32786e4fa0fc664ab0ad36de
diff --git a/libc/tools/genversion-scripts.py b/libc/tools/genversion-scripts.py
index 0a98994..5410580 100755
--- a/libc/tools/genversion-scripts.py
+++ b/libc/tools/genversion-scripts.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 # This tool is used to generate the version scripts for libc and libm
 # for every architecture.