tools: fix shebang in packer/setup.sh

Change shebang in script from `/bin/bash` to `/usr/bin/env bash`,
which fixed problems when `bash` is not available in standard location,
e.g., on non-Linux OSes such as FreeBSD.

dist/tools/packer/setup.sh

@@ -1,4 +1,4 @@
-#!/bin/bash -eux
+#!/usr/bin/env bash -eux
 
 # inspired by https://github.com/boxcutter/ubuntu