Fixing python3 shebang according to PEP 394

Signed-off-by: alexander <alexander.efremov@arm.com>
Change-Id: I7d46ebbbd6a026cf9ee36acafe74039962c756e5
diff --git a/build_default.py b/build_default.py
old mode 100644
new mode 100755
index b61da13..3bb91b1
--- a/build_default.py
+++ b/build_default.py
@@ -1,4 +1,4 @@
-#!env/bin/python3
+#!/usr/bin/env python3
 
 #  Copyright (c) 2021 Arm Limited. All rights reserved.
 #  SPDX-License-Identifier: Apache-2.0