Remove some designated initializers

With the switch to C++20, the rules became more strict, and we can no
longer initialize base classes. Make them comments instead.

(BTW
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2287r1.html
this offers some new syntax for this use-case. Hopefully this will be
adopted and we can eventually use it.)

src/libexpr/primops/fetchTree.cc

@@ -240,7 +240,7 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
                     .method = unpack ? FileIngestionMethod::Recursive : FileIngestionMethod::Flat,
                     .hash = *expectedHash,
                 },
-                .references = {}
+                /* .references = */ {}
             });
 
         if (state.store->isValidPath(expectedPath)) {