toJSON: report error position for fancier output

Given flake:

```nix
{ description = "nix json error provenance";
  inputs = {};
  outputs = { self }: {
    jsonFunction = _: "function";
    json = builtins.toJSON (_: "function");
  };
}

```
- Before:

```console
❯ nix eval --json .#jsonFunction
error: cannot convert a function to JSON
```

- After:

```console
❯ nix eval --json .#jsonFunction
error: cannot convert a function to JSON

       at /nix/store/b7imf1c2j4jnkg3ys7fsfbj02s5j0i4f-source/testflake/flake.nix:4:5:

            3|   outputs = { self }: {
            4|     jsonFunction = _: "function";
             |     ^
            5|     json = builtins.toJSON (_: "function");
```

1 files changed, 1 insertions, 1 deletions

diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc
index 25d0fa3ba..ac48682c7 100644
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/src/nix-instantiate/nix-instantiate.cc
@@ -52,7 +52,7 @@ void processExpr(EvalState & state, const Strings & attrPaths,
             if (output == okXML)
                 printValueAsXML(state, strict, location, vRes, std::cout, context);
             else if (output == okJSON)
-                printValueAsJSON(state, strict, vRes, std::cout, context);
+                printValueAsJSON(state, strict, vRes, noPos, std::cout, context);
             else {
                 if (strict) state.forceValueDeep(vRes);
                 std::cout << vRes << std::endl;