JSON: print paths as strings without copying them to the store

Makes `printValueAsJSON` not copy paths to the store for `nix eval
--json`, `nix-instantiate --eval --json` and `nix-env --json`.

Fixes https://github.com/NixOS/nix/issues/5612

src/nix/eval.cc

@@ -116,7 +116,7 @@ struct CmdEval : MixJSON, InstallableCommand
 
         else if (json) {
             JSONPlaceholder jsonOut(std::cout);
-            printValueAsJSON(*state, true, *v, pos, jsonOut, context);
+            printValueAsJSON(*state, true, *v, pos, jsonOut, context, false);
         }
 
         else {