add pos to EvalState::forceValue

[kamadorueda]

• This way we improve error messages
  on infinite recursion
• Demo:

  let
    x = builtins.fetchMercurial x;
  in
  x

• Before:

  $ nix-instantiate --show-trace --strict
  error: infinite recursion encountered

• After:

  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

• This is my first contribution so I picked an issue that was relatively easy in order to familiarize with the project

Mentions: #3505

[nrdxp]

Thank you for this!