Make HttpBinaryCacheStore::narFromPath() run in constant memory

This reduces memory consumption of

  nix copy --from https://cache.nixos.org --to ~/my-nix /nix/store/95cwv4q54dc6giaqv6q6p4r02ia2km35-blender-2.79

from 176 MiB to 82 MiB. (The remaining memory is probably due to xz
decompression overhead.)

Issue https://github.com/NixOS/nix/issues/1681.
Issue https://github.com/NixOS/nix/issues/1969.