tests.pkg-config: extend current Nixpkgs configuration

This was enabling aliases on ofborg.

pkgs/top-level/pkg-config/tests.nix

@@ -1,6 +1,6 @@
 # cd nixpkgs
 # nix-build -A tests.pkg-config
-{ lib, stdenv, ... }:
+{ lib, config, stdenv, ... }:
 
 let
   # defaultPkgConfigPackages test needs a Nixpkgs with allowUnsupportedPlatform
@@ -10,7 +10,7 @@ let
   allPkgs = import ../default.nix {
     system = stdenv.hostPlatform.system;
     localSystem = stdenv.buildPlatform.system;
-    config = {
+    config = config // {
       allowUnsupportedSystem = true;
     };
     overlays = [];