fix(gs/system): Explicitly add all default i3 keybindings

Something I changed - perhaps removing lib.mkOptionDefault - caused
the *default* i3 keybindings (things like switching workspaces and
toggling the split layout which I had not added myself) not to get
added. This adds them back.

Change-Id: I301a346eabb4f7fc4499b60a4c26956fb1e08b0d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2218
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/glittershark/system/home/machines/chupacabra.nix

@@ -59,4 +59,6 @@ in {
       ssh mugwump sudo $system/bin/switch-to-configuration switch
     '')
   ];
+
+  xsession.windowManager.i3.config.keybindings.F7 = "exec lock";
 }