Parent tasks in workflows with chunks do not have chunk suffixes

Sentry Issue: ARKINDEX-BACKEND-43

AssertionError: Missing parent ufcn_line_historical_630ff8 for task kaldi_horae_resized_bf557e_1
(18 additional frame(s) were not displayed)
...
  File "contextlib.py", line 74, in inner
    return func(*args, **kwds)
  File "ponos/models.py", line 364, in build_tasks
    for slug, recipe in self.recipes.items()
  File "django/utils/functional.py", line 48, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "ponos/models.py", line 325, in recipes
    _, recipes = parse_recipe(self.recipe)
  File "ponos/recipe.py", line 84, in parse_recipe
    assert parent in tasks, "Missing parent {} for task {}".format(parent, slug)

The created recipe is visible here: https://preprod.arkindex.teklia.com/admin/ponos/workflow/68856a67-c4e6-4cd1-9cff-a4691a419e8b/change/

Tasks are properly created as ufcn_1, ufcn_2 etc but their parents lack the suffixes (meaning kaldi_1 depends on ufcn although it does not exist), causing errors in task dependencies when starting the workflow. Caused by this change