Fix exportReferencesGraph when given store subpath

With Nix 2.3, it was possible to pass a subpath of a store path to
exportReferencesGraph:

	with import <nixpkgs> {};

	let
	  hello = writeShellScriptBin "hello" ''
	    echo ${toString builtins.currentTime}
	  '';
	in

	writeClosure [ "${hello}/bin/hello" ]

This regressed with Nix 2.4, with a very confusing error message, that
presumably indicates it was unintentional:

	error: path '/nix/store/3gl7kgjr4pwf03f0x70dgx9ln3bhl7zc-hello/bin/hello' is not in the Nix store

(cherry picked from commit 0774e8ba33)

src/libstore/parsed-derivations.cc

@@ -186,7 +186,7 @@ std::optional<nlohmann::json> ParsedDerivation::prepareStructuredAttrs(Store & s
         for (auto i = e->begin(); i != e->end(); ++i) {
             StorePathSet storePaths;
             for (auto & p : *i)
-                storePaths.insert(store.parseStorePath(p.get<std::string>()));
+                storePaths.insert(store.toStorePath(p.get<std::string>()).first);
             json[i.key()] = pathInfoToJSON(store,
                 store.exportReferences(storePaths, inputPaths));
         }