support for patching nixpkgs instance used by home manager

- introduce support for patching the used nixpkgs instance via
  `applyPatches`, see https://ertt.ca/nix/patch-nixpkgs/
- use patched nixpkgs instance in mkHomeManagern (TODO: use it in more
  places)
- add patch for nodejs-20 test failure in darwin (currently just in
  staging)

flake.nix

@@ -46,6 +46,16 @@
     let
       # FIXME: allow different systems
       systems = utils.lib.system;
+
+      # ability to extend nixpkgs with patches, e.g. from PRs or staging. See https://ertt.ca/nix/patch-nixpkgs/
+      mkNixpkgs-patched =
+        system:
+        (import nixpkgs { inherit system; }).applyPatches {
+          name = "nixpkgs-patched";
+          src = nixpkgs;
+          patches = [ ./patches/nixos-nixpkgs-362c2bc76d359b908ad1389ee28c23ee0e862324.patch ];
+        };
+
       # necessary to make the top-level inputs available to system configuration
       # inspired by https://jade.fyi/blog/flakes-arent-real/
       inputInjection =
@@ -78,7 +88,7 @@
       mkHomeManager =
         confName: user: system: # unfortunately, home-manager configs are still system-specific
         home-manager.lib.homeManagerConfiguration {
-          pkgs = nixpkgs.legacyPackages.${system};
+          pkgs = import (mkNixpkgs-patched system) { inherit system; };
           modules = [
             {
               imports = [