set NIX_PATH, enables use of <nixpkgs>

infra/common/nixos/default.nix

@@ -1,8 +1,8 @@
-{ lib, ... }:
+{ lib, pkgs, ... }:
 
 let
   inherit (lib) mkDefault;
-
+  nixPath = "/run/current-system/nixpkgs";
 in
 {
   imports = [
@@ -16,6 +16,15 @@ in
   system.stateVersion = "24.05"; # do not change
   nixpkgs.hostPlatform = mkDefault "x86_64-linux";
 
+  # use flake's nixpkgs over channels
+  nix.nixPath = [ "nixpkgs=${nixPath}" ];
+  system.extraSystemBuilderCmds = ''
+    ln -sv ${pkgs.path} $out/nixpkgs
+  '';
+  systemd.tmpfiles.rules = [
+    "L+ ${nixPath} - - - - ${pkgs.path}"
+  ];
+
   ## This is just nice to have, but it is also particularly important for the
   ## Forgejo CI runners because the Nix configuration in the actions is directly
   ## taken from here.