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).

cmd/serve/restic/restic-test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Test all the remotes against restic integration test
 # Run with: screen -S restic-test -L ./restic-test.sh