[v5.8] Packit: Add cautionary note to ephemeral copr job

[lsm5]

The packit copr jobs are consumed by podman-machine-os release process so any change to the copr targets should be mindful of this fact.

(cherry picked from commit 8ce58d3)

Checklist

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• Certify you wrote the patch or otherwise have the right to pass it on as an open-source patch by signing all
  commits. (git commit -s). (If needed, use git commit -s --amend). The author email must match
  the sign-off email address. See CONTRIBUTING.md
  for more information.
• Referenced issues using Fixes: #00000 in commit message (if applicable)
• Tests have been added/updated (or no tests are needed)
• Documentation has been updated (or no documentation changes are needed)
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• Release note entered in the section below (or None if no user-facing changes)

Does this PR introduce a user-facing change?

None