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`.

example/btrfs-subvolumes.nix

@@ -35,12 +35,12 @@
                 };
                 # Mountpoints inferred from subvolume name
                 "/home" = {
-                  mountOptions = ["compress=zstd"];
+                  mountOptions = [ "compress=zstd" ];
                 };
                 "/nix" = {
-                  mountOptions = ["compress=zstd" "noatime"];
+                  mountOptions = [ "compress=zstd" "noatime" ];
                 };
-                "/test" = {};
+                "/test" = { };
               };
             };
           }