Update shell script shebangs

While `/bin/bash` is the default in many systems, it cannot be relied
upon for _all_ systems, particularly ones such as MacOS where the
default `/bin/bash` is a very old version. Instead, use the `bash`
executable from the system's PATH.

setup.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 # Accept the `--remove` argument to delete symlinks of the files to be copied

src/.bash_profile

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 ### environment vars
 # Ensure $HOME/.bin exists, add it to PATH

src/.bashrc

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # If not running interactively, don't do anything
 [ -z "$PS1" ] && return