workspace: Don't unfreeze layout during overview animation

During overview closing animation the allocation expands more or less
every frame. Unfreezing the layout then leads to windows snapping to
their new position if e.g. a window was closed which doesn't look nice.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3216

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1550>
(cherry picked from commit d82dcd6f)