nix-daemon: Disable path info cache

This is useless because the client also caches path info, and can
cause problems for long-running clients like hydra-queue-runner
(i.e. it may return cached info about paths that have been
garbage-collected).

src/libutil/sync.hh

@@ -33,6 +33,7 @@ public:
 
     Sync() { }
     Sync(const T & data) : data(data) { }
+    Sync(T && data) noexcept : data(std::move(data)) { }
 
     class Lock
     {