wrapper-manager-fds: improve devshell for non-flake users

shell.nix

@@ -1,10 +1,19 @@
-{ pkgs ? import <nixpkgs> { } }:
+let
+  sources = import ./npins;
+in
+{ pkgs ? import sources.nixos-unstable { } }:
 
 pkgs.mkShell {
-  inputsFrom = with pkgs; [ nix ];
   packages = with pkgs; [
     npins
     treefmt
     nixpkgs-fmt
+
+    hugo
+    asciidoctor
+
+    # For easy validation of the test suite.
+    yajsv
+    jq
   ];
 }