Support an "umbrella" GitHub status check

Right now each stack shows up as a separate GitHub check on a PR, which makes it hard/impossible to set these checks as “required” in a monorepo with many independent stacks: each PR would only trigger a subset of stacks and the checks for the untriggered stacks would never show up, hence blocking the PR from merging. This is a common problem for many CI systems and a common solution is to implement an “umbrella” check that always shows up and will fail if any job fails.