tools/toolbox: Set up first toolbox as default

Setting up more than one gnome-shell toolbox is uncommon, and users
shouldn't have to specify --set-default for other tools to pick the
right default.

It would be possible to detect the number of containers that were
created from the shell toolbox image, but the far easier option is
to just set up the first container as default.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2935>

tools/toolbox/create-toolbox.sh

@@ -104,6 +104,9 @@ unset TEMP
 NAME=$DEFAULT_NAME
 LOCALES=()
 
+# set up first toolbox as default
+[ ! -f $CONFIG_FILE ] && SET_DEFAULT=1
+
 while true; do
   case "$1" in
     -n|--name)