Function nix_compat::derivation::ca_kind_prefix

fn ca_kind_prefix(ca_hash: &CAHash) -> &'static str

For a CAHash, return the “prefix” used for NAR purposes. For CAHash::Flat, this is an empty string, for CAHash::Nar, it’s “r:”. Panics for other CAHash kinds, as they’re not valid in a derivation context.