Fix absolute shebangs in install scripts (#16135)

* Fix absolute shebang in /bin/flutter

* Fix absolute shebang in bin/internal/update_dart_sdk.sh
diff --git a/bin/flutter b/bin/flutter
index 2ef55db..079c3f9 100755
--- a/bin/flutter
+++ b/bin/flutter
@@ -1,4 +1,4 @@
-#!/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.
diff --git a/bin/internal/update_dart_sdk.sh b/bin/internal/update_dart_sdk.sh
index 7fdf325..7dd160d 100755
--- a/bin/internal/update_dart_sdk.sh
+++ b/bin/internal/update_dart_sdk.sh
@@ -1,4 +1,4 @@
-#!/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.