`__contentAddressed` support

[SomeoneSerge]

nix-diff seems to struggle parsing CA derivations:

❯ 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

CC @mschwaig #OceanSprint #OceanSprint2025