Fix absolute shebangs in install scripts (#16135)

* Fix absolute shebang in /bin/flutter

* Fix absolute shebang in bin/internal/update_dart_sdk.sh

bin/flutter

-#!/bin/bash
+#!/usr/bin/env bash
 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.

bin/internal/update_dart_sdk.sh

-#!/bin/bash
+#!/usr/bin/env bash
 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.