travis: also check NixOS options

This will at least catch simple errors in default values and is fairly
cheap, in terms of resource and time consumption, and adds very little
additional output unless there's a failure.

maintainers/scripts/travis-nox-review-pr.sh

@@ -26,6 +26,9 @@ elif [[ $1 == build ]]; then
     echo "=== Checking tarball creation"
     nix-build pkgs/top-level/release.nix -A tarball
 
+    echo "=== Checking NixOS options"
+    nix-build --show-trace nixos/release.nix -A options
+
     if [[ $TRAVIS_PULL_REQUEST == false ]]; then
         echo "=== Not a pull request"
     else