build: use /usr/bin/env to call python

Hardcoding the path to python instead of env is not recommended.  This
can break the build in some Linux environments that link /usr/bin/python
to python3.

Change-Id: I4c62eb391acc6d468ce24ac8640834e965e76e78
diff --git a/core/checktree b/core/checktree
index b0b9cfa..2872683 100755
--- a/core/checktree
+++ b/core/checktree
@@ -1,4 +1,4 @@
-#!/usr/bin/python -E
+#!/usr/bin/env python -E
 
 import sys, os, re