dev/ci: generate notifications for release nightly builds

Created by: bobheadxi

Release nightly healthchecks, if failed, will now try to tag either a Slack user based on captainGitHubUsername or the dev experience support handle. I noticed this request in RFC 590

We cannot use Slack usernames for tagging because doing so is not supported by Buildkite/Slack.

Test plan

export SLACK_INTEGRATION_TOKEN=...
go run enterprise/dev/ci/gen-pipeline.go -overrideRunType=1 -yaml

You get:

notify:
- if: build.state == "failed"
  slack:
    channels:
    - '#buildkite-main'
    message: cc <@U02UJSAEQP9>

The above is @michaellzc , current release captain