fix: changed shebang to make scripts more platform independent (#2893)

See for example this thread:

https://stackoverflow.com/questions/21612980/why-is-usr-bin-env-bash-superior-to-bin-bash

on why its better to use '#!/usr/bin/env bash' instead.
Without this change these scripts will not run on some
platforms. This is not a breaking change for the
platforms it already works on.

app/run-ble-test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright (c) 2023 The ZMK Contributors
 # SPDX-License-Identifier: MIT

docs/src/templates/setup.sh.mustache

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright (c) 2020 The ZMK Contributors
 # SPDX-License-Identifier: MIT