fetchTree: add pos to EvalState::forceValue
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: #3505

NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix