Temporarily do Darwin builds from a different Nixpkgs branch

release.nix

@@ -76,7 +76,8 @@ let
 
     build = pkgs.lib.genAttrs systems (system:
 
-      with import <nixpkgs> { inherit system; };
+      # FIXME: temporarily use a different branch for the Darwin build.
+      with import (if system == "x86_64-darwin" then <nixpkgs-darwin> else <nixpkgs>) { inherit system; };
 
       releaseTools.nixBuild {
         name = "nix";
@@ -110,7 +111,8 @@ let
 
     binaryTarball = pkgs.lib.genAttrs systems (system:
 
-      with import <nixpkgs> { inherit system; };
+      # FIXME: temporarily use a different branch for the Darwin build.
+      with import (if system == "x86_64-darwin" then <nixpkgs-darwin> else <nixpkgs>) { inherit system; };
 
       let
         toplevel = builtins.getAttr system jobs.build;