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/make-content-addressed.cc

@@ -56,7 +56,7 @@ std::map<StorePath, StorePath> makeContentAddressed(
                     .method = FileIngestionMethod::Recursive,
                     .hash = narModuloHash,
                 },
-                .references = std::move(refs),
+                /* .references = */ std::move(refs),
             },
             Hash::dummy,
         };