Create repos_to_delete if not exists

What does this MR do and why?

To delete repos, ensure the repos_to_delete directory exists. If it does not exist it will create the directory

How to set up and validate locally

1. Remove the repos_to_delete. rm -rf gitlab-development-kit/zoekt-data/development/index/repos_to_delete
2. Create a delete_repo task.
3. Verify that the task gets processed successfully and repos_to_delete exists.

Closes #51 (closed)