cups: support prefixed pkg-config

Otherwise configure fails with the following error:
- configure: error: Need pkg-config to enable libusb support.

pkgs/misc/cups/default.nix

···
 
   propagatedBuildInputs = [ gmp ];
 
+  configurePlatforms = [ "build" "host" ];
   configureFlags = [
     "--localstatedir=/var"
     "--sysconfdir=/etc"