Use larger runners for nightly jobs

[mhucka]

The nightly jobs are failing due to running out of disk space. (It's the same problem we had recently with the CI checks workflow.) This PR updates the jobs to use the larger self-hosted runners from Google ML Velocity.

In addition, this PR makes some simplifications to the workflows to get rid of some of the overelaborate things I did before, and make minor other changes to keep with best practices such as putting timeouts on jobs.