Nightly jobs should only trigger daily if there have been SCM changes

Looking back at our Jenkinsfile I just realized why we're running Nightlies every day for all branches regardless of changes. We use this stanza:

  triggers {
    cron('@daily')
  }

It should probably be a daily check to see if the branch has changed. That would alleviate both my own concerns about stale feature branches and some of the infra concerns about how many build slots we've been using.

If possible, having a "build once a week (or month) even if there have been no changes" would be nice, but I haven't looked at the Jenkins Pipeline DSL in a while so I can't remember if the set of triggers is an OR.

relevant docs:

https://jenkins.io/doc/book/pipeline/syntax/#triggers