actions: fix fork PR runs

[DL6ER]

What does this implement/fix?

Skip both the attestation and the deployment steps for fork-based PRs. As they have no access to secrets, these steps will always fails. This is also the reason why the recent external contribution https://github.com/pi-hole/FTL/pull/1971 failed.

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• [x] The code change is tested and works locally.
• [x] I based my code and PRs against the repositories developmental branch.
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