Fix sg broken release process

Created by: jhchabran

Because at some point, when they are too many releases, GitHub stops making latest point at them, the current release process is broken. Until I made a commit on the main branch, none of the successful GHA created release were flagged as the latest one, which always caused scripts to download a version from 19 days ago, which was the last that was successfully marked as latest.

This commit introduces an additional step that commits automatically on each release in sourcegraph/sg README to make sure we always have a proper sha1 associated with that release and can always have latest properly updated.

See successful run here: https://github.com/sourcegraph/sourcegraph/runs/4692599347?check_suite_focus=true and its release https://github.com/sourcegraph/sg/releases/tag/2022-01-03-15-30-acc7e848