Fix subseasonal startdate recipe splitting when giving individual dates

@vtorralba noticed a bug in the subseasonal case. When providing multiple start dates in this way the recipe is not divided correctly:

  Time:
    sdate: 
      - 20240823 #%Y%m%d
      - 20240830

This is because of a bug in the code of tools/divide_recipe.R that considers only the current start_date in the loop, and generates a vector of only one start date even when there are multiple, resulting in only one recipe.

This MR fixes this bug.

Victòria