The s-string task calls `dist/s_string` with the -r flag to check if we can remove whitelisted words from s_string.ok.

Currently this runs every 90 days, but we'd be better served by running it on each PR, and that way whenever a whitelisted word is no longer needed due to PR changes we can update s_string.ok at the same time.

To correct this we'll move the task out of infrequent-checks, drop the 90 day cadence, and add the pull_request tag so it runs on all PRs.