style: Apply nixfmt to trivial Nix files

ALl the ones except for build-image.nix are considered trivial. On the
latter, nixfmt makes some useful changes but by-and-large it is not
ready for that code yet.

tools/nixery/shell.nix

@@ -14,14 +14,11 @@
 
 # Configures a shell environment that builds required local packages to
 # run Nixery.
-{pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import <nixpkgs> { } }:
 
 let nixery = import ./default.nix { inherit pkgs; };
 in pkgs.stdenv.mkDerivation {
   name = "nixery-dev-shell";
 
-  buildInputs = with pkgs;[
-    jq
-    nixery.nixery-build-image
-  ];
+  buildInputs = with pkgs; [ jq nixery.nixery-build-image ];
 }