Use overlaid pkgs on old nix

compat/default.nix

@@ -0,0 +1 @@
+{ ... }: (builtins.getFlake (toString ../.)).legacyPackages.${builtins.currentSystem}

default.nix

@@ -0,0 +1,9 @@
+{ ... }:
+let
+  self = (import (let lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+  in fetchTarball {
+    url =
+      "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+    sha256 = lock.nodes.flake-compat.locked.narHash;
+  }) { src = ./.; }).defaultNix;
+in self // self.legacyPackages.${builtins.currentSystem}

modules/nix.nix

@@ -1,6 +1,6 @@
 { lib, inputs, ... }: {
   nix = rec {
-    nixPath = lib.mkForce [ "nixpkgs=/etc/nixpkgs" ];
+    nixPath = lib.mkForce [ "nixpkgs=/etc/self/compat" ];
     binaryCaches = [ "https://cache.nixos.org" ];
 
     registry.self.flake = inputs.self;
@@ -23,4 +23,5 @@
     requireSignedBinaryCaches = true;
   };
   environment.etc.nixpkgs.source = inputs.nixpkgs;
+  environment.etc.self.source = inputs.self;
 }