dev/buildchecker: unify reporting runs, remove top-level slack token requirement
requested to merge 06-30-dev/buildchecker_unify_reporting_runs_remove_top-level_slack_token_requirement into main
Created by: bobheadxi
History reporting is an expensive operation - we pull what could be many pages of builds from Buildkite in order to generate a report. This PR removes the duplicated reporting so that only one run is required, and also removes the slack token that is actually only required for checks.
Test plan
yolo merge and fix if broken turns out i can do a test run: https://github.com/sourcegraph/sourcegraph/runs/7139571549?check_suite_focus=true