freshBootstrapTools.bootstrapTools: extract as a package

pkgs/stdenv/darwin/bootstrap-tools.nix

···
+{
+  lib,
+  stdenv,
+  bootstrapTools,
+  unpack,
+}:
+
+builtins.derivation {
+  inherit (stdenv.hostPlatform) system;
+
+  name = "bootstrap-tools";
+  builder = "${unpack}/bin/bash";
+
+  args = [
+    "${unpack}/bootstrap-tools-unpack.sh"
+    bootstrapTools
+  ];
+
+  PATH = lib.makeBinPath [
+    (builtins.placeholder "out")
+    unpack
+  ];
+
+  allowedReferences = [ "out" ];
+}

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

···
     '';
   };
 
-  bootstrapTools = derivation {
-    inherit (stdenv.hostPlatform) system;
-
-    name = "bootstrap-tools";
-    builder = "${bootstrapFiles.unpack}/bin/bash";
-
-    args = [
-      "${bootstrapFiles.unpack}/bootstrap-tools-unpack.sh"
-        bootstrapFiles.bootstrapTools
-    ];
-
-    PATH = lib.makeBinPath [
-      (placeholder "out")
-      bootstrapFiles.unpack
-    ];
-
-    allowedReferences = [ "out" ];
+  bootstrapTools = pkgs.callPackage ./bootstrap-tools.nix {
+    inherit (bootstrapFiles) bootstrapTools unpack;
   };
 
   test = derivation {