Change hardcoded bash shebang to env

- dotnet/aspnetcore#695
- support various `bash` installation locations
- in particular, enable building on FreeBSD

build.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 if test `uname` = Darwin; then
     cachedir=~/Library/Caches/KBuild