Rework ci labels

[kwannoel]

1. Instead of ci/main-cron/skip-ci + ci/run-main-cron, just have ci/main-cron/run-all and ci/main-cron/run-selected.
2. run-all will run all steps in the main-cron workflow.
3. run-selected will only run selected steps.

Same applies to PR workflow.

Additionally, just auto run all the build steps, so user doesn't have to manually add them.

Buildkite conditional for main-cron

(build.pull_request.id != null && (build.pull_request.labels includes "ci/main-cron/run-all" || build.pull_request.labels includes "ci/main-cron/run-selected")  || build.tag != null