fix(grfn/system): Don't use whitby as substituter, only builder

I've found that this is the best way to convince nix to actually prefer
cache.nixos.org - it tries to use whitby as a builder, then if the store
path is already built it just downloads it.

Change-Id: I4c78079bfb0013155feb2d39f60d99779123109e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5972
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/system/system/machines/ogopogo.nix

@@ -7,7 +7,7 @@
     ../modules/xserver.nix
     ../modules/fonts.nix
     ../modules/sound.nix
-    # ../modules/tvl.nix
+    ../modules/tvl.nix
     ../modules/development.nix
     ../modules/work/kolide.nix
   ];

users/grfn/system/system/modules/tvl.nix

@@ -18,7 +18,6 @@
     settings = {
       substituters = [
         "https://cache.nixos.org"
-        "ssh://nix-ssh@whitby.tvl.fyi"
       ];
       trusted-substituters = [
         "https://cache.nixos.org"