bin/windup using Bash syntax causes error when run with /bin/sh

Our bin/windup script uses Bash syntax, e.g. ADDONS_DIR=().
But it states #!/bin/sh on the first line.
This leads to syntax error:

/tmp/windup/unzipped/bin/windup: 27: /tmp/windup/unzipped/bin/windup: Syntax error: "(" unexpected

We can either use #!/bin/bash
or not use the Bash syntax.