buildchecker: enforce branch protections for admins as well

Created by: bobheadxi

I made an accidental push to main because I am an admin on the repo 😬 I think this config was wiped by Buildchecker unintentionally, this PR makes sure we set protections to admins as well

Thankfully the pushed change was a minor refactor (https://github.com/sourcegraph/sourcegraph/commit/6c0ead544e4f3b21de91f7ca2e7e8c8d7f77a818) but will keep an eye on the CI build

Test plan

n/a, unit tests still pass