Use dummy store for XML conversion

Nix trying to open the store inside a derivation leads to issues on systems using NIS (https://github.com/nix-community/home-manager/issues/3605).

Since we are only doing evaluation, we don't need a store so we can use the special dummy:// store introduced in Nix 2.4 (https://nixos.org/manual/nix/unstable/release-notes/rl-2.4.html). (This will work even for people who are staying on 2.3 since we are using pkgs.nix directly.)