The workspace minimap is much less prominent than the old workspace
switcher, and serves primarily as an indicator.
That means that duplicating it on secondary monitors (if workspaces
on non-primaries are enabled) is harder to mistake for per-monitor
workspaces, so make some people happy by including the minimap on
every monitor.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1735>