add pos to EvalState::forceValue

- This way we improve error messages
  on infinite recursion
- Demo:
  ```nix
  let
    x = builtins.fetchMercurial x;
  in
  x
  ```
- Before:
  ```bash
  $ nix-instantiate --show-trace --strict
  error: infinite recursion encountered
  ```
- After:
  ```bash
  nix-instantiate --show-trace --strict
  error: infinite recursion encountered

       at /data/github/kamadorueda/test/default.nix:2:7:

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

Mentions: #3505

0 files changed, 0 insertions, 0 deletions