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/path-info.cc

@@ -31,7 +31,7 @@ std::optional<ContentAddressWithReferences> ValidPathInfo::contentAddressWithRef
             assert(references.count(path) == 0);
             return TextInfo {
                 th,
-                .references = references,
+                /* .references = */ references,
             };
         },
         [&](const FixedOutputHash & foh) -> ContentAddressWithReferences {
@@ -43,7 +43,7 @@ std::optional<ContentAddressWithReferences> ValidPathInfo::contentAddressWithRef
             }
             return FixedOutputInfo {
                 foh,
-                .references = {
+                /* .references = */ {
                     .others = std::move(refs),
                     .self = hasSelfReference,
                 },