download.cc: fix readCallback return type (int -> size_t)
download.cc: fix readCallback return type (int -> size_t)

I'm not sure if curl ever asks for enough data at once
for truncation to occur but better safe than sorry.

rycee/nix,vcunat/nix,peti/nix,zimbatm/nix,NixOS/nix,layus/nix,vcunat/nix,NixOS/nix,pikajude/nix,vcunat/nix,peti/nix,layus/nix,zimbatm/nix,ehmry/nix,ehmry/nix,dtzWill/nix,peti/nix,NixOS/nix,peti/nix,dtzWill/nix,layus/nix,pikajude/nix,pikajude/nix,dtzWill/nix,rycee/nix,ehmry/nix,rycee/nix,dtzWill/nix,layus/nix,ehmry/nix,layus/nix,ehmry/nix,rycee/nix,NixOS/nix,zimbatm/nix,ehmry/nix,dtzWill/nix,vcunat/nix,NixOS/nix,zimbatm/nix,rycee/nix,peti/nix