ci: allow fork checkout in submission validator

[duongja]

This fixes the Validate Submission workflow for fork-based solution PRs.

Current failure on LP-0002 submission PR #115:

• workflow: Validate Submission
• run: https://github.com/logos-co/lambda-prize/actions/runs/30554437421
• failing step: Checkout PR head (untrusted data, never executed)
• error: Refusing to check out fork pull request code from a pull_request_target workflow

The workflow already treats the PR-head checkout as untrusted data and does not execute code from it. It checks out the fork only so the trusted base-branch validation script can inspect submitted markdown content and changed files.

Change:

• set allow-unsafe-pr-checkout: true on the PR-head actions/checkout@v4 step.

After this merges, fork-based solution PRs such as #115 can be rerun by the normal validator workflow.

[github-actions]

⚠️ Validation script did not produce output.

[duongja]

Closing this as a duplicate of #109.

#109 is the same fix for the same pull_request_target fork-checkout guard that is currently blocking fork-based solution PR validation, including LP-0002 #115.