toXML: display errors position

- This change applies to builtins.toXML and inner workings
- Proof of concept:
  ```nix
  let e = builtins.toXML e; in e
  ```
- Before:
  ```
  $ nix-instantiate --eval poc.nix
  error: infinite recursion encountered
  ```
- After:
  ```
  $ nix-instantiate --eval poc.nix
  error: infinite recursion encountered

       at /data/github/kamadorueda/nix/poc.nix:1:9:

            1| let e = builtins.toXML e; in e
             |
  ```

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..6ff3d2f19 100644
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/src/nix-instantiate/nix-instantiate.cc
@@ -50,7 +50,7 @@ void processExpr(EvalState & state, const Strings & attrPaths,
             else
                 state.autoCallFunction(autoArgs, v, vRes);
             if (output == okXML)
-                printValueAsXML(state, strict, location, vRes, std::cout, context);
+                printValueAsXML(state, strict, location, vRes, std::cout, context, noPos);
             else if (output == okJSON)
                 printValueAsJSON(state, strict, vRes, std::cout, context);
             else {