Be flexible regarding bash path in dltools.sh

Bash isn't always at /bin/bash. On nixos, it might be a nix store path.
On macOS, users might want to use a more up-to-date version of bash than
what ships with the system (bash 3, from 2007).

Locate bash from PATH instead.

dltools.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 SCRIPT_DIR=$(dirname "${BASH_SOURCE[0]}")
 BOLD=$(tput bold)