buildchecker: allow multiple channel announcements

Created by: bobheadxi

buildchecker currently posts to #buildkite-main so that locks and unlocks can be seen inline with other failure notifications. However, some feedback suggests not everyone tunes into #buildkite-main so this adds the capability for us to adjust the Slack webhooks variable to include multiple Slack webhooks for announcements.

I'm planning on adding a #prod-eng-announce announcement when this gets merged