ci: add nightly scheduled GitHub Actions job (#4432)
ci: add nightly scheduled GitHub Actions job (#4432)

Summary:
This extends our current GitHub Actions CI workflow to run nightly at
05:00 PST (06:00 PDT, 13:00 UTC). The [docs for scheduled events][1]
indicate that such a job will run on the default (master) branch. This
job should run with write access to the build cache.

[1]: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows#scheduled-events

Test Plan:
Note that the workflow parses and normal CI still runs on this job.
We’ll wait for tomorrow to see if it actually runs.

wchargin-branch: ci-gh-nightly

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard