`CA:` field is missing in `.narinfo` files

[vobloeb]

Unlike cache.nixos.org, nix-serve's .narinfo files have no CA: field.

As the result, FOD's downloaded from nix-serve are not marked as FOD locally.

It is a minor issue which have subtle consequences you may hit years after: it breaks nothing until you perform an action which depends on whether a derivation FOD or not (e.g. you may want to prevent FODs from being GC'ed and suddenly you have no way to distinguish them reliably)