Use eval-store in more places

In particular, this now works:

  $ nix path-info --eval-store auto --store https://cache.nixos.org nixpkgs#hello

Previously this would fail as it would try to upload the hello .drv to
cache.nixos.org. Now the .drv is instantiated in the local store, and
then we check for the existence of the outputs in cache.nixos.org.