fetchTree: add pos to EvalState::forceValue

- This way we improve error messages
  on infinite recursion
- Demo:
  ```nix
  let x = builtins.fetchTree {
    type = "git";
    inherit 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:3:10:

            2|   type = "git";
            3|   inherit x;
             |          ^
            4| };
  ```

Mentions: NixOS#3505

src/libexpr/primops/fetchTree.cc

@@ -121,7 +121,7 @@ static void fetchTree(
 
         for (auto & attr : *args[0]->attrs) {
             if (attr.name == state.sType) continue;
-            state.forceValue(*attr.value);
+            state.forceValue(*attr.value, *attr.pos);
             if (attr.value->type() == nPath || attr.value->type() == nString) {
                 auto s = state.coerceToString(*attr.pos, *attr.value, context, false, false);
                 attrs.emplace(attr.name,