[scripts][bash] use /usr/bin/env to find bash for FreeBSD

Change-Id: I377efbccc233d85cba8ef794fbf2b1669e82e0ea

scripts/make-parallel

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 
 # Copyright 2016 The Fuchsia Authors
 #