fix(nix): Install system-manager

I missed this since I haven't deal with overlays that much before.

nix/systems/nix/default.nix

@@ -12,6 +12,9 @@ in
           arch = "x86_64-linux";
           stable = nixpkgs-stable;
           unstable = nixpkgs-unstable;
+          extraOverlays = [
+            system-manager.overlays.default
+          ];
           nur = nur;
         }
       ) system pkgs pkgs-unstable;

nix/systems/nix/potato/default.nix

@@ -17,6 +17,7 @@
       };
       systemPackages =
         common.packages ++ [
+          pkgs.system-manager
         ];
     };