sysctl: Nixify sysctl.sh

Use "#!/usr/bin/env bash" instead of "#!/bin/bash". Needed for testing
in nix environments as they only provide /usr/bin/env at the standard
location.

Signed-off-by: Joel Granados <[e-mail]>

tools/testing/selftests/sysctl/sysctl.sh

···
-#!/bin/bash
+#!/usr/bin/env bash
 # SPDX-License-Identifier: GPL-2.0-or-later OR copyleft-next-0.3.1
 # Copyright (C) 2017 Luis R. Rodriguez <[e-mail]>