stdenv-darwin: allow easier testing of bootstrap tools

This un-hardcodes the bootstrap tools passed into the Darwin stdenv and
thus allows us to quickly iterate on improving the design of the full
bootstrap process. We can easily change the contents of the bootstrap
tools and evaluate an entire bootstrap all the way up to real packages.

pkgs/stdenv/darwin/make-bootstrap-tools.nix

@@ -1,4 +1,6 @@
-with import ../../top-level/all-packages.nix { system = "x86_64-darwin"; };
+{ system ? builtins.currentSystem }:
+
+with import ../../top-level/all-packages.nix { inherit system; };
 
 rec {
   # We want coreutils without ACL support.
@@ -169,7 +171,15 @@ rec {
     '';
   };
 
-  unpack = stdenv.mkDerivation {
+  bootstrapFiles = {
+    sh      = "${build}/on-server/sh";
+    bzip2   = "${build}/on-server/bzip2";
+    mkdir   = "${build}/on-server/mkdir";
+    cpio    = "${build}/on-server/cpio";
+    tarball = "${build}/on-server/bootstrap-tools.cpio.bz2";
+  };
+
+  unpack = stdenv.mkDerivation (bootstrapFiles // {
     name = "unpack";
 
     # This is by necessity a near-duplicate of unpack-bootstrap-tools.sh. If we refer to it directly,
@@ -216,14 +226,8 @@ rec {
       EOF
     '';
 
-    tarball = "${build}/on-server/bootstrap-tools.cpio.bz2";
-
-    mkdir = "${build}/on-server/mkdir";
-    bzip2 = "${build}/on-server/bzip2";
-    cpio  = "${build}/on-server/cpio";
-
     allowedReferences = [ "out" ];
-  };
+  });
 
   test = stdenv.mkDerivation {
     name = "test";
@@ -283,4 +287,12 @@ rec {
       $out/bin/hello
     '';
   };
+
+  # The ultimate test: bootstrap a whole stdenv from the tools specified above and get a package set out of it
+  test-pkgs = let
+    stdenv = import ./. { inherit system bootstrapFiles; };
+  in import ../../top-level/all-packages.nix {
+    inherit system;
+    bootStdenv = stdenv.stdenvDarwin;
+  };
 }