scripts: fix interpreter for bash (#12549)

Many of our scripts have a non-portable interpreter line for bash and
use bash-specific variables like `BASH_SOURCE`. Update the interpreter
line to be portable between various Linuxes and macOS without
complaint from posix shell users.

demo/csi/hostpath/run.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Run the hostpath plugin and create some volumes, and then claim them.
 set -e