spin: switch back to dropbox mirror

see #38542 for details

pkgs/development/tools/analysis/spin/default.nix

···
-{ stdenv, lib, requireFile, makeWrapper, yacc, gcc
+{ stdenv, lib, fetchurl, makeWrapper, yacc, gcc
 , withISpin ? true, tk, swarm, graphviz }:
 
 let
···
   version = "6.4.8";
   url-version = stdenv.lib.replaceChars ["."] [""] version;
 
-  src = requireFile {
-    name = "spin${url-version}.tar.gz";
-    sha256 = "1rpazi5fj772121cn7r85fxypmaiv0x6x2l82b5y1xqzyf0fi4ph";
-    message = ''
-      reCAPTCHA is preventing us to download the file for you.
-      Please download it at http://spinroot.com/spin/Src/index.html
-      and add it to the nix-store using nix-prefetch-url.
-    '';
+  src = fetchurl {
+    # The homepage is behind CloudFlare anti-DDoS protection, which blocks cURL.
+    # Dropbox mirror from developers:
+    # https://www.dropbox.com/sh/fgzipzp4wpo3qc1/AADZPqS4aoR-pjNF6OQXRLQHa
+    url = "https://www.dropbox.com/sh/fgzipzp4wpo3qc1/AADya1lOBJZDbgWGrUSq-dfHa/spin${url-version}.tar.gz?raw=1";
+    sha256 = "1rvamdsf0igzpndlr4ck7004jw9x1bg4xyf78zh5k9sp848vnd80";
   };
 
   nativeBuildInputs = [ makeWrapper ];
   buildInputs = [ yacc ];
 
   sourceRoot = "Spin/Src${version}";
-
-  unpackPhase = ''
-    # The archive is compressed twice
-    gunzip -c $src > spin.tar.gz
-    tar -xzf spin.tar.gz
-  '';
 
   installPhase = ''
     install -Dm755 spin $out/bin/spin