dhall-to-nix: Quote field selection symbols

Another one I missed, when you have a field selector, you want to
quote it, in case it has some symbols nix does not know how to handle.

`x.Foo/bar`  will now be `x."Foo/bar"`, which is valid nix.

dhall-nix/src/Dhall/Nix.hs

@@ -696,7 +696,7 @@ dhallToNix e =
             _ -> return (unionChoice k Nothing)
     loop (Field a (Dhall.Core.fieldSelectionLabel -> b)) = do
         a' <- loop a
-        return (a' @. b)
+        return (Fix (Nix.NSelect a' (mkDoubleQuoted b :| []) Nothing))
     loop (Project a (Left b)) = do
         a' <- loop a
         let b' = fmap StaticKey (toList b)