fetch: nicer infinite recursion errors

- This change applies to builtins.fetchurl and builtins.fetchTarball
- PoC: `let x = builtins.fetchurl 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.fetchurl x; in x
             |         ^
  ```

Mentions: NixOS#3505

src/libexpr/primops/fetchTree.cc

@@ -189,7 +189,7 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
     std::optional<std::string> url;
     std::optional<Hash> expectedHash;
 
-    state.forceValue(*args[0]);
+    state.forceValue(*args[0], pos);
 
     if (args[0]->type() == nAttrs) {