Fix github_api helper to use BUILDKITE_REPO

[wzieba]

• it should use BUILDKITE_REPO instead of BUILDKITE_PULL_REQUEST_REPO because we want to communicate with GitHub API not necessarily on a PR-associated branch
• probably use regex expression from https://github.com/Automattic/a8c-ci-toolkit-buildkite-plugin/pull/124/files#diff-704b8fc3d47317b87aeb9552bfa16ec27caf723ccf9380e72fe508b6c7c6c207R7-R11 to determine repo owner and name. It supports both https and git@ coordinates

[AliSoftware]

@wzieba Ah, I missed this, I already made a PR to fix it in #127 😅

We could recycle this issue to instead say "adopt github_api helper in upload_sarif_to_github implementation instead though, as #127 didn't cover that, and I'm guessing you probably only want to look at that once you've confirmed your initial issue on trunk branch of client repos? 🤷
Or if you update upload_sarif_to_github right away we can keep this issue closed and done; whatever you prefer.