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.cc

@@ -1287,7 +1287,7 @@ drvName, Bindings * attrs, Value & v)
                 .method = method,
                 .hash = h,
             },
-            .references = {},
+            /* .references = */ {},
         });
         drv.env["out"] = state.store->printStorePath(outPath);
         drv.outputs.insert_or_assign("out",
@@ -2103,7 +2103,7 @@ static void addPath(
                     .method = method,
                     .hash = *expectedHash,
                 },
-                .references = {},
+                /* .references = */ {},
             });
 
         if (!expectedHash || !state.store->isValidPath(*expectedStorePath)) {

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)) {