`__contentAddressed` support

`nix-diff` seems to struggle parsing CA derivations: ```console ❯ nix-diff \ "$(nix-instantiate "<nixpkgs>" -A hello)" \ "$(nix-instantiate "<nixpkgs>" --arg config "{contentAddressedByDefault = true;}" -A hello)" nix-diff: user error (Could not parse a derivation from this file: /nix/store/7vkm78pnc8njx7c3sz3l7d7bpcm5kwv9-hello-2.12.1.drv) laut on  main [!+?] via 🐍 took 2s ❯ nix-instantiate "<nixpkgs>" --arg config "{contentAddressedByDefault = true;}" -A hello /nix/store/7vkm78pnc8njx7c3sz3l7d7bpcm5kwv9-hello-2.12.1.drv ```