binary download: Use $NIX_CURL_FLAGS

As in https://github.com/NixOS/nixpkgs/blob/5c0816567d6b99bd2ef7c8ae5744f80a6a0372c4/pkgs/build-support/fetchurl/builder.sh#L17
Merge pull request #372 from wmertens/patch-4

binary download: Use $NIX_CURL_FLAGS