Use more portable shebang for dev scripts

/bin/bash, while common, is not part of the unix standard, and does not
exist on some operating systems (such as NixOS). /usr/bin/env, on the
other hand, is standardized, and thus should exist on all systems.

bw-dev

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # exit on errors
 set -e

update.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 # determine inital and target versions