all: use consistent shebang line across all shell scripts

Change-Id: I4aac882b1b618a388d0748a427dc998203d3a1b2
Reviewed-on: https://go-review.googlesource.com/113856Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/mkalldocs.sh

-#!/bin/bash
+#!/usr/bin/env bash
 # Copyright 2012 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.

src/naclmake.bash

-#!/bin/bash
+#!/usr/bin/env bash
 # Copyright 2016 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.

src/nacltest.bash

-#!/bin/bash
+#!/usr/bin/env bash
 # Copyright 2014 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.

src/runtime/mknacl.sh

-#!/bin/bash
+#!/usr/bin/env bash
 # Copyright 2013 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.