overviewControls: Reduce spacing around the workspaces minimap

overviewControls: Reduce spacing around the workspaces minimap

With a previous commit we've made the overall dynamic spacing dynamic, but
the spacing is now a bit much when it comes to the space between search and
workspaces minimap, just as workspaces minimap and large workspaces.

The  minimap should somewhat "squeeze into the empty space" here, so
reduce the spacings above and below it a bit.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3278>