style: reformat with nixpkgs-fmt

system/shared/nixpkgs.nix

@@ -1,7 +1,5 @@
-{ }: {
-  config = {
-    allowUnfree = true;
-  };
+{}: {
+  config = { allowUnfree = true; };
 
   overlays = [ (import ./overlays.nix) ];
 }

system/shared/overlays.nix

@@ -1,4 +1 @@
-self: super: {
-  customVim = with self; {
-  };
-}
+self: super: { }