xcbuild: fix sdk dir name issue

Previously, this error was coming up in xcbuild:

ld: malformed 32-bit x.y.z version number: 1068638

It’s an interesting error because it only happens with certain hashes for the
nixpkgs sdk. For instance, on latest nixpkgs unstable channel, the hash for the
xcbuild sdk is:

/nix/store/w6mwbdaz9calyii0fyxspl51f1068638-nix.nixpkgs.sdk

that is an issue we pass -isysroot ${sdk} to clang where it will interpret that
hanging "1068638". It would probably go away as soon as the hash changes but
this hacky fix will solve the problem.

pkgs/development/tools/xcbuild/platform.nix

@@ -228,6 +228,6 @@ stdenv.mkDerivation {
 
     mkdir -p $out/Developer/SDKs/
     cd $out/Developer/SDKs/
-    ln -s ${sdk}
+    cp -r ${sdk} nix.nixpkgs.sdk
   '';
 }