tools/toolbox: Explicitly use /bin/bash

The script uses bash-specific language extensions which cause syntax
errors when run with /bin/sh -> /bin/dash.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3186>

tools/toolbox/create-toolbox.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 # vi: sw=2 ts=4
 
 set -e