fetchTree: add pos to EvalState::forceValue

- This way we improve error messages
  on infinite recursion
- Demo:
  ```nix
  let x = builtins.fetchTree x;
  in x
  ```
- Before:
  ```bash
  $ nix-instantiate --extra-experimental-features flakes --strict
  error: infinite recursion encountered
  ```
- After:
  ```bash
  $ nix-instantiate --extra-experimental-features flakes --strict
  error: infinite recursion encountered

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

            1| let x = builtins.fetchTree x;
             |         ^
            2| in x
  ```

Mentions: NixOS#3505

src/libexpr/primops/fetchTree.cc

@@ -97,7 +97,7 @@ static void fetchTree(
     fetchers::Input input;
     PathSet context;
 
-    state.forceValue(*args[0]);
+    state.forceValue(*args[0], pos);
 
     if (args[0]->type() == nAttrs) {
         state.forceAttrs(*args[0], pos);