Add validation to enforce skip_cleanup=false cannot be used with backends

[SarahFrench]

(Work split out of #36848)

This PR:

• Adds validation to stop users setting skip_cleanup=false in a run block with a backend block
  • This is because using a backend block implies skip_cleanup=true
  • We could pivot to making backend and skip_cleanup mutually exclusive?
• Adds tests for that validation, and also to show it's ok to set skip_cleanup=true in a run block with a backend block

[github-actions]

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