lib.generators: made toLua accept derivations too

While trying to extend generateLuarocksConfig, I had infinite loops caused by toLua considering derivations as attrSets

lib/generators.nix

@@ -525,6 +525,8 @@ ${expr "" v}
           "(${v.expr})"
         else if v == { } then
           "{}"
+        else if libAttr.isDerivation v then
+          ''"${toString v}"''
         else
           "{${introSpace}${concatItems (
             lib.attrsets.mapAttrsToList (key: value: "[${builtins.toJSON key}] = ${toLua innerArgs value}") v