Refactor scheduled pipeline jobs to avoid using dnf

Right now the scheduled job we run uses cki-project's builder-eln image; previously (!2593 (merged)) we had used builder-rawhide.

Every time the job runs, it uses dnf to install these packages: python3-gitlab git openssh-clients dnf-utils gnupg2

That's great, until there is some packaging error and it blocks the jobs: https://gitlab.com/cki-project/kernel-ark/-/jobs/7165703673

Instead, whatever container image we use for these jobs should already have the necessary packages/bits installed. If we add these packages as part of some cki-project container image definition then we'll have a dependable image as cki-project won't publish new images unless all its defined packages installed successfully.

AC:

• ability to use
  • cki-tools image: cki-tools!1303 (merged)
  • builder-rawhide image: containers!559 (merged)
• refactor scheduled jobs: !3207 (merged)
  • schedules should use environments for their protected variables
  • scheduled jobs should have dry-run mode available on MRs that do everything up to the final push