tree-wide: replace /bin/bash with /usr/bin/env bash

The latter is more portable, while the former only works on systems
where /bin/bash exists (or is symlinked appropriately).

bin/build-xgo-cgofuse.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 docker build -t rclone/xgo-cgofuse https://github.com/winfsp/cgofuse.git
 docker images