Cache octokit in GitHub workflows

[jimchamp]

Closes #9250

Caches @octokit/action for applicable GitHub workflows.

Technical

Caches octokit in the same way that it is cached in #9311. The key is reused for each octokit cache entry. If additional JS dependencies are added to any workflow that caches octokit, the cache key may need to be updated for that particular workflow.

Testing

Merge this into your forked repo's master branch, then trigger one of the workflows. If there is no cache hit, wait until the workflow is finished, then trigger it again. Expect the npm install step to be skipped on the second run.

Screenshot

Stakeholders

[jimchamp]

I wonder if the stale_assignee_digest workflow can be removed?