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/libstore/binary-cache-store.cc

@@ -313,7 +313,7 @@ StorePath BinaryCacheStore::addToStoreFromDump(Source & dump, std::string_view n
                     .method = method,
                     .hash = nar.first,
                 },
-                .references = {
+                /* .references = */ {
                     .others = references,
                     .self = false,
                 },
@@ -431,7 +431,7 @@ StorePath BinaryCacheStore::addToStore(
                     .method = method,
                     .hash = h,
                 },
-                .references = {
+                /* .references = */ {
                     .others = references,
                     .self = false,
                 },