tesh: actually pick the path to python3 that we have

diff --git a/tools/tesh/tesh.py b/tools/tesh/tesh.py
index 7e1ace3..c8a0d8a 100755 (executable)
--- a/tools/tesh/tesh.py
+++ b/tools/tesh/tesh.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#! @PYTHON_EXECUTABLE@
 # -*- coding: utf-8 -*-
 """