Move gnupg import inside signing script fixture

[gerrod3]

This should resolve the CI failures for plugins that don't require gnupg for their tests.

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

See: Pull Request Walkthrough

[patchback]

Backport to 3.108: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.108/2a223a768a17559d87897fe71483f35e0ee2b050/pr-7618

Backported as #7619

🤖 @patchback
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