Replace /bin/bash with env call (#2409)

Fixes build on FreeBSD, which uses /usr/local/bin/bash,
and also is just a more idiomatic way to handle shells.

bootstrap.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Copyright 2023 LiveKit, Inc.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");