Fix nix-copy-closure test

Fixes

  client# error: size mismatch importing path ‘/nix/store/ywf5fihjlxwijm6ygh6s0a353b5yvq4d-libidn2-0.16’; expected 0, got 120264

This is mostly an artifact of the NixOS VM test environment, where the
Nix database doesn't contain hashes/sizes.

http://hydra.nixos.org/build/53537471

src/libstore/store-api.cc

@@ -547,6 +547,7 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
     if (!info->narHash && dontCheckSigs) {
         auto info2 = make_ref<ValidPathInfo>(*info);
         info2->narHash = hashString(htSHA256, *sink.s);
+        if (!info->narSize) info2->narSize = sink.s->size();
         info = info2;
     }