curl: Change default src url (close #12851)

- Changes the default src url away from http://curl.haxx.se
to an official mirror because haxx.se resolves to https and
bootstrap curl doesn't have https support.
Please see https://github.com/NixOS/nixpkgs/issues/12816
for more info.

pkgs/tools/networking/curl/default.nix

@@ -19,7 +19,7 @@ stdenv.mkDerivation rec {
   name = "curl-7.45.0";
 
   src = fetchurl {
-    url = "http://curl.haxx.se/download/${name}.tar.bz2";
+    url = "http://ngcobalt13.uxnr.de/mirror/curl/${name}.tar.bz2";
     sha256 = "1slq5c0v9wa8hajgimhkxhvsrd07jmih8sa3gjsl597qp5k4w5b5";
   };