scripts: Use shebang /usr/bin/env bash

Not all distros have `/bin/bash`, e.g. NixOS.

Fixes: #3450

Signed-off-by: Sebastian Hasler <sebastian.hasler@stuvus.uni-stuttgart.de>

tools/osbuilder/rootfs-builder/alpine/rootfs_lib.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Copyright (c) 2018 HyperHQ Inc.
 #

tools/osbuilder/rootfs-builder/rootfs.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Copyright (c) 2018 Intel Corporation
 #