dev shell: add nixos-config to NIX_PATH

Makes inspecting the system to be built from a local checkout with nix repl easier.
Inspecting the system still needs root permissions, though, because ENC
files in /etc/nixos/ are not accessible by normal users.

shell.nix

@@ -8,7 +8,12 @@ let
     ":"
     (lib.mapAttrsToList (name: channel: "${name}=${channel}") channels);
 
+  nixosRepl = pkgs.writeShellScriptBin "nixos-repl" "nix repl nixos-repl.nix";
+
 in pkgs.mkShell {
   name = "fc-nixos";
-  NIX_PATH="fc=${toString ./.}:${nixPathUpstreams}";
+  NIX_PATH="fc=${toString ./.}:${nixPathUpstreams}:nixos-config=/etc/nixos/configuration.nix";
+  shellHook = ''
+    export PATH=$PATH:${nixosRepl}/bin
+  '';
 }