Function writes 

pub fn writes(
    runner: &str,
    job_runner: Option<&str>,
    repository: &str,
    trigger: &RunTrigger,
    branches: &[String],
    writers: BuildCacheWriters,
) -> Writes

Whether runner’s job may write, from what GitHub says: the runner it placed the job on (job_runner, which a job cannot forge as it can its runner’s log), and its run’s trigger. A push, scheduled or manually started run of repository itself on one of branches may. With writers at PullRequests, so may a pull_request run of one of the repository’s own branches that people started, once GitHub also says each of its pull requests was opened by one (opened_by_user). Undecided while GitHub places the job on no runner.