Faster flake.lock parsing

This PR reduces the creation of short-lived basic_json objects while
parsing flake.lock files. For large flake.lock files (~1.5MB) I was
observing ~60s being spent for trivial nix build operations while
after this change it is now taking ~1.6s.

(cherry picked from commit 7fd0de3)
(cherry picked from commit f94fb63)

src/libexpr/flake/lockfile.cc

@@ -89,7 +89,7 @@ LockFile::LockFile(const nlohmann::json & json, const Path & path)
                 std::string inputKey = i.value();
                 auto k = nodeMap.find(inputKey);
                 if (k == nodeMap.end()) {
-                    auto nodes = json["nodes"];
+                    auto & nodes = json["nodes"];
                     auto jsonNode2 = nodes.find(inputKey);
                     if (jsonNode2 == nodes.end())
                         throw Error("lock file references missing node '%s'", inputKey);