Increase commit chunkiness

[ahouseholder]

All the workflows are currently operating on the main branch. This results in a lot of small commits every time a workflow runs.

There are a few related tasks here

• adapt the search and update_summaries jobs in SearchRepos workflow to do their work in a branch or branches, then squash-merge the results back to main. Remove the working branch when done.

• subtask of the above, or could be treated separately: Once update_summaries has done its job, the intermediate per-search result json files can be deleted. So they can exist on the working branch, but would never need to make it to main. Only summaries would get into main. Note, however, that this will require changes to generate_summaries so that we can continue to do monthly and yearly summaries too. (It's not as simple as adding a remove-all-non-summaries method.)

• adapt the deep_dive and repo2vulid jobs in SearchRepos workflow to do their work in a branch or branches, then squash-merge the results back to main. Unlike the search/summaries items above, we want to retain both the repo and vul-id centric views, so in this case there is no post-action cleanup to be done.

[ahouseholder]

I have a demonstration of the branch -> do work -> squash-merge -> delete branch workflow in https://github.com/ahouseholder/stunning-umbrella

[ahouseholder]

See also #17 which talks about splitting the data into a separate repository from the code that generates it. It's probably a good idea to consider either doing #1 and #17 together, or at least do #17 first.