Expose `ourpkgs` as `config.passthru.ourpkgs` for use by tests.

We currently expose `ourpkgs` to nixos modules via an `ourpkgs` argument using
`_module.args`. However, `_module.args` is not available in the final
configuraiton. Thus, to allow tests to access the value, we also expose
is via passthru.

nixos/modules/packages.nix

 # A NixOS module which exposes custom packages to other modules.
 { pkgs, ...}:
-{
+let
+  ourpkgs = pkgs.callPackage ../../nixos/pkgs {};
+in {
   config = {
     # Expose `nixos/pkgs` as a new module argument `ourpkgs`.
-    _module.args.ourpkgs = pkgs.callPackage ../../nixos/pkgs {};
+    _module.args.ourpkgs = ourpkgs;
+    # Also expose it as a config setting, for usage by tests,
+    # since the `_module` config is not exposed in the result.
+    passthru.ourpkgs = ourpkgs;
   };
 }