build(container): avoid forcing shebang

Let’s let the system decide which binary to use instead of forcing one. For
example, on macOS, /bin/bash points to the default bash version, which is 3.2…
I think that speaks for itself! :D

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/distribution/build_container.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright (c) Meta Platforms, Inc. and affiliates.
 # All rights reserved.