replace shebang '/bin/bash' for '/usr/bin/env bash'

Allows better portability of bash scripting between Linux
distributions.

Change-Id: Ia0ec079581398bcffbf20bb8206860bcec0e6e7a
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
diff --git a/job/tf-build-for-lava/build_test.sh b/job/tf-build-for-lava/build_test.sh
index c16128e..bcc98d0 100755
--- a/job/tf-build-for-lava/build_test.sh
+++ b/job/tf-build-for-lava/build_test.sh
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Copyright (c) 2019, Arm Limited. All rights reserved.
 #