Fix mcfgthreads lookup (input-output-hk#2057)

Author: hamishmack

builder/comp-builder.nix

@@ -620,7 +620,7 @@ let
               lib.optionals (builtins.compareVersions defaults.ghc.version "9.4.1" >= 0)
                 [ buildPackages.gcc-unwrapped
                   # Find the versions of mcfgthreads used by stdenv.cc
-                  (pkgs.threadsCrossFor or (x: windows.mcfgthreads) stdenv.cc.version).package
+                  (pkgs.threadsCrossFor or (x: { package = pkgs.windows.mcfgthreads; }) pkgs.stdenv.cc.version).package
                 ])}; do
           find "$p" -iname '*.dll' -exec ln -s {} $out/bin \;
         done

lib/system-nixpkgs-map.nix

@@ -10,7 +10,7 @@ let
   # On windows systems we need these to be propagatedBuildInputs so that the DLLs will be found.
   gcclibs = if pkgs.stdenv.hostPlatform.isWindows then [
     # Find the versions of mcfgthreads used by stdenv.cc
-    (pkgs.threadsCrossFor or (x: pkgs.windows.mcfgthreads) pkgs.stdenv.cc.version).package
+    (pkgs.threadsCrossFor or (x: { package = pkgs.windows.mcfgthreads; }) pkgs.stdenv.cc.version).package
     # If we just use `pkgs.buildPackages.gcc.cc` here it breaks the `th-dlls` test. TODO figure out why exactly.
     (pkgs.buildPackages.runCommand "gcc-only" { nativeBuildInputs = [ pkgs.buildPackages.xorg.lndir ]; } ''
       mkdir $out