chore: don't report pull request build status to SauceLabs

As of today SauceLabs doesn't have any notion of git branches which means that any build - main branch or PR - will update "latest build" status. In practice it means that a failing PR build will make it look like the framework build is failing - a failed build status will be displayed on GitHub.

This PR changes the situation be reporting build status back to Sauce only on master branch builds.