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/bisect-go-rclone.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # An example script to run when bisecting go with git bisect -run when
 # looking for an rclone regression