Change deb postrm shebang to /bin/bash

Fixes microsoft#6758

resources/linux/debian/postrm.template

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/bash
 #
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # Licensed under the MIT License. See License.txt in the project root for license information.