forceValue: make pos mandatory

- Make passing the position to `forceValue` mandatory,
  this way we remember people that the position is
  important for better error messages
- Add pos to all `forceValue` calls

src/nix-build/nix-build.cc

@@ -318,7 +318,7 @@ static void main_nix_build(int argc, char * * argv)
 
         for (auto & i : attrPaths) {
             Value & v(*findAlongAttrPath(*state, i, *autoArgs, vRoot).first);
-            state->forceValue(v);
+            state->forceValue(v, v.determinePos(noPos));
             getDerivations(*state, v, "", *autoArgs, drvs, false);
         }
     }