Create repos_to_delete if not exists

There are a lot of these types of open /data/index/repos_to_delete/14649832.delete: no such file or directory errors from the zoekt indexer when it tries to process the delete_repo task. We think the problem is missing the repos_to_delete directory.

Proposal: use mkdir -p for repos_to_delete to make sure when %id.delete files are getting created there exists repos_to_delete