flake.nix: Set nixpkgs.config.path

flake.nix

@@ -48,6 +48,10 @@
                   system.nixos.versionSuffix =
                     ".${final.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}.${self.shortRev or "dirty"}";
                   system.nixos.revision = final.mkIf (self ? rev) self.rev;
+
+                  # NOTE: This assumes that `nixpkgs.config` is _not_ used when
+                  #       nixpkgs.pkgs is set OR _module.args.pkgs is set.
+                  nixpkgs.config.path = self.outPath;
                 }
               ];
           });

nixos/modules/misc/nixpkgs.nix

@@ -59,6 +59,8 @@ let
     inherit (cfg) config overlays localSystem crossSystem;
   };
 
+  # NOTE: flake.nix assumes that nixpkgs.config is only used with ../../..
+  #       as nixpkgs.config.path should be equivalent to ../../..
   finalPkgs = if opt.pkgs.isDefined then cfg.pkgs.appendOverlays cfg.overlays else defaultPkgs;
 
 in