Change '/bin/bash' shebangs to '/usr/bin/env bash'

This improves portability across different unix-like operating systems
by using bash from the PATH, instead of bash from a hardcoded location.

Signed-off-by: Harry Callahan <hcallahan@lowrisc.org>

examples/simple_system/spike-simple-system.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -u
 set -e