build: use consistent shebang line

Fixes #13948.

Change-Id: I37d734c1241f3d72d8fe33dfdf79b573e5476b1f
Reviewed-on: https://go-review.googlesource.com/18643
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/src/bootstrap.bash b/src/bootstrap.bash
index 1b5ba7c..7947e84 100755
--- a/src/bootstrap.bash
+++ b/src/bootstrap.bash
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Copyright 2015 The Go Authors.  All rights reserved.
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.