binutils: Do not needless redownload sources cascading more rebuilds

Unfortunately this is a crude hack that we use the same binutils source
everywhere in the bootstrap chain.

pkgs/development/tools/misc/binutils/default.nix

@@ -19,10 +19,11 @@ in
 stdenv.mkDerivation rec {
   name = targetPrefix + basename;
 
-  src = fetchurl {
+  # HACK to ensure that we preserve source from bootstrap binutils to not rebuild LLVM
+  src = stdenv.__bootPackages.binutils-unwrapped.src or (fetchurl {
     url = "mirror://gnu/binutils/${basename}.tar.bz2";
     sha256 = "028cklfqaab24glva1ks2aqa1zxa6w6xmc8q34zs1sb7h22dxspg";
-  };
+  });
 
   patches = [
     # Turn on --enable-new-dtags by default to make the linker set