style: Re-apply style normalisation

Originally this was manually applied with;

```sh
nixpkgs-fmt **.nix && statix fix
```

but I overlooked the fact that `**.nix` would only expand to files in
the root (I should have used `**/*.nix`).

Previous commit adds `nix fmt` support which passes `.` to `nixpkgs-fmt`
(if no other path is explicitly specified when running `nix fmt`).

This commit includes the changes made by running `nix fmt`.

tests/lib.nix

@@ -100,7 +100,7 @@
         documentation.enable = false;
 
         nix.settings = {
-          substituters = lib.mkForce [];
+          substituters = lib.mkForce [ ];
           hashed-mirrors = null;
           connect-timeout = 1;
         };