nix copy: Build derivations

Fixes

  $ nix copy .#hydraJobs.vendoredCrates --to /tmp/nix
  error: path '/nix/store/...' is not valid

src/nix/copy.cc

@@ -40,6 +40,8 @@ struct CmdCopy : StorePathsCommand
             .shortName('s')
             .description("whether to try substitutes on the destination store (only supported by SSH)")
             .set(&substitute, Substitute);
+
+        realiseMode = Build;
     }
 
     std::string description() override