util/abuild: Use env to find bash

FreeBSD doesn't have bash in /bin, so use env to find it.  This is
already done in many other scripts that are used in the actual build
path.

Change-Id: If6fb6bc3c55835e2144599fea1cdb2f7abefb0fc
Signed-off-by: Martin Roth <martinr@coreboot.org>
Reviewed-on: https://review.coreboot.org/28364
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Alex Thiessen <alex.thiessen.de+coreboot@gmail.com>

util/abuild/abuild

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 #  coreboot autobuild
 #