stdenv/darwin: add bootstrap tools for aarch64-darwin

Taken from PR #12355; other parts are still being discussed.

pkgs/stdenv/darwin/default.nix

@@ -5,15 +5,15 @@
   if localSystem.isAarch64 then
     let
       fetch = { file, sha256, executable ? true }: import <nix/fetchurl.nix> {
-        url = null; # to be built by hydra and injected here
+        url = "http://tarballs.nixos.org/stdenv-darwin/aarch64/20acd4c4f14040485f40e55c0a76c186aa8ca4f3/${file}";
         inherit (localSystem) system;
         inherit sha256 executable;
       }; in {
-        sh      = fetch { file = "sh";    sha256 = "0000000000000000000000000000000000000000000000000000"; };
-        bzip2   = fetch { file = "bzip2"; sha256 = "0000000000000000000000000000000000000000000000000000"; };
-        mkdir   = fetch { file = "mkdir"; sha256 = "0000000000000000000000000000000000000000000000000000"; };
-        cpio    = fetch { file = "cpio";  sha256 = "0000000000000000000000000000000000000000000000000000"; };
-        tarball = fetch { file = "bootstrap-tools.cpio.bz2"; sha256 = "0000000000000000000000000000000000000000000000000000"; executable = false; };
+        sh      = fetch { file = "sh";    sha256 = "17m3xrlbl99j3vm7rzz3ghb47094dyddrbvs2a6jalczvmx7spnj"; };
+        bzip2   = fetch { file = "bzip2"; sha256 = "1khs8s5klf76plhlvlc1ma838r8pc1qigk9f5bdycwgbn0nx240q"; };
+        mkdir   = fetch { file = "mkdir"; sha256 = "1m9nk90paazl93v43myv2ay68c1arz39pqr7lk5ddbgb177hgg8a"; };
+        cpio    = fetch { file = "cpio";  sha256 = "17pxq61yjjvyd738fy9f392hc9cfzkl612sdr9rxr3v0dgvm8y09"; };
+        tarball = fetch { file = "bootstrap-tools.cpio.bz2"; sha256 = "1v2332k33akm6mrm4bj749rxnnmc2pkbgcslmd0bbkf76bz2ildy"; executable = false; };
       }
   else
     let