[fixed] Make GHA workflow to retrieve latest promote-able SHA from master (#79559)
Relates to #76700
**Overview**: Wrote GHA to get the latest commit SHA. Another component of the script is pushing this SHA to the viable/strict branch, which I will test on pytorch/pytorch-canary.
Todo in the next PR: add comment explaining cron, replace package installation statements with txt file
**Test Plan:** Monitor github actions results to see if the SHA printed is correct by running GHA on pytorch/pytorch-canary. The successful test workflow is [here](https://github.com/pytorch/pytorch-canary/runs/6888486129?check_suite_focus=true).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79559
Approved by: https://github.com/janeyx99
