Input::getAccessor(): Get the fingerprint from the final accessor

Fixes an issue reported by @erikarvstedt where
InputAccessor::fetchToStore() wouldn't cache the store path the first
time.

src/libfetchers/cache.cc

@@ -130,6 +130,7 @@ struct CacheImpl : Cache
         std::string_view key,
         std::string_view value) override
     {
+        debug("upserting fact '%s' -> '%s'", key, value);
         _state.lock()->upsertFact.use()
             (key)
             (value).exec();