flake.nix: Set formatter

This enables `nix fmt`, though it won't be practically usable without
also reformatting all files, which is done in a following commit.

ci/default.nix

@@ -52,6 +52,7 @@ let
     in
     {
       shell = treefmtEval.config.build.devShell;
+      pkg = treefmtEval.config.build.wrapper;
     };
 
 in

flake.nix

@@ -133,6 +133,8 @@
           default = import ./shell.nix { inherit system; };
         });
 
+      formatter = forAllSystems (system: (import ./ci { inherit system; }).fmt.pkg);
+
       /**
         A nested structure of [packages](https://nix.dev/manual/nix/latest/glossary#package-attribute-set) and other values.