lib.licenses: use && for exception licenses

Seems preferable to iterating over a two-element list.
licenseType.exception is only used by lib.licenses.WITH, which I don't
see any usage of in tree - but thought I might as well.

lib/licenses/helpers.nix

@@ -11,10 +11,7 @@ let
       else
         throw "Unknown license operator"
     else if license.licenseType == "exception" then
-      AND evaluateSubProperty [
-        license.license
-        license.exception
-      ]
+      evaluateSubProperty license.license && evaluateSubProperty license.exception
     else if license.licenseType == "plus" then
       evaluateSubProperty license.license
     else