Fix boost::bad_format_string exception in builtins.addErrorContext (#9291)

* Fix boost::bad_format_string exception in builtins.addErrorContext

The message passed to addTrace was incorrectly being used as a format
string and this this would cause an exception when the string contained
a '%', which can be hit in places where arbitrary file paths are
interpolated.

* add test

(cherry picked from commit 61d6fe059e959455e156c1d57bb91155d363e983)
Change-Id: Idd671127a9c1ccc8b94e58e727632fcc064f3cbe

src/libexpr/primops.cc

@@ -818,7 +818,7 @@ static void prim_addErrorContext(EvalState & state, const PosIdx pos, Value * *
         auto message = state.coerceToString(pos, *args[0], context,
                 "while evaluating the error message passed to builtins.addErrorContext",
                 false, false).toOwned();
-        e.addTrace(nullptr, message, true);
+        e.addTrace(nullptr, hintfmt(message), true);
         throw;
     }
 }