Update chunkSize to the suggested value

This was a suggested course of action in a review in one of our earlier
commits, https://github.com/NixOS/nix/pull/3801#discussion_r457557079

src/libstore/local-store.cc

@@ -1069,7 +1069,7 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, const string & name,
        or the original source is empty */
     while (dump.size() < settings.narBufferSize) {
         auto oldSize = dump.size();
-        constexpr size_t chunkSize = 1024;
+        constexpr size_t chunkSize = 65536;
         auto want = std::min(chunkSize, settings.narBufferSize - oldSize);
         dump.resize(oldSize + want);
         auto got = 0;