fix: portable shebangs

bin/common_defs.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 DEBUG="${DEBUG:-0}"
 if [ "$DEBUG" -eq 1 ]; then

bin/common_defs2.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 ## NOTE: those are defined separately from `common_defs.sh` because
 ## they require `common_functions.sh` to be sourced prior to sourcing

bin/common_functions.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Echo to stderr on errors
 echoerr() { echo "ERROR: $*" 1>&2; }