Use org.gnome.mutter instead of org.gnome.shell.overrides.

I noticed that when I use multiple monitors and go to the overview screen switching workspaces does not make my secondary monitor follow the primary monitor. All my open apps are shown in the secondary monitor.

I'm not sure if this is the way to solve it but by replacing shell.overrides by mutter the setting workspaces-only-on-primary is being used.

This is by no means backward compatible so if it needs to be backwards compatible another solution may be needed.