tests.pkg-config.defaultPkgConfigPackages: fix cross compilation

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

···
   # unsupported platform problems.
   allPkgs = import ../default.nix {
     system = stdenv.hostPlatform.system;
-    localSystem = stdenv.hostPlatform.system;
+    localSystem = stdenv.buildPlatform.system;
     config = {
       allowUnsupportedSystem = true;
     };