style: Apply nixpkgs-fmt and fix

Apply standard formatting and some statix conventions using;

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

With the intent of making contribution a bit easier and reducing mental
load in hand formatting (in the same vein as [black]).

[black]: https://github.com/psf/black#the-uncompromising-code-formatter

ci.nix

@@ -1,6 +1,7 @@
 let
-  pkgs = import <nixpkgs> {};
-in {
+  pkgs = import <nixpkgs> { };
+in
+{
   test = pkgs.writeScript "test" ''
     #!/bin/sh
     nix-build "${toString ./tests}";