finalized GHA for promoting to viable/strict (#79649)
Relates to #76700
**Overview:** Finalized GHA to push commits to viable/strict. Added `git push` statement to GHA that updates the viable/strict branch, modified checkout repo step.
**Test Plan:** Tested on `pytorch/pytorch-canary` [here.](https://github.com/pytorch/pytorch-canary/runs/6922229055?check_suite_focus=true ) Need to edit branch protection rules in https://fburl.com/code/w3tiegy6
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79649
Approved by: https://github.com/seemethere
