run-libcore-tests.py: Fix hash-bang to work on LUCI bots.

Change-Id: I66e3a4715e9940c2c67f24065dde5db83120e27c
diff --git a/tools/run-libcore-tests.py b/tools/run-libcore-tests.py
index ebcf7d3..e6fbc59 100755
--- a/tools/run-libcore-tests.py
+++ b/tools/run-libcore-tests.py
@@ -1,4 +1,8 @@
-#!/bin/python3
+#!/usr/bin/env python3
+#
+# [VPYTHON:BEGIN]
+# python_version: "3.8"
+# [VPYTHON:END]
 #
 # Copyright (C) 2021 The Android Open Source Project
 #