workspacesView: Update scroll position but not workspace on allocate

Not updating the scroll position on allocate caused a glitch, where a
window on a workspace that is not the first one would would be
transparent for one frame when showing the overview.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3507


(cherry picked from commit 1685c779)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1564>