[#373] Add repository guard to scheduled link-check workflow

[Labreo]

Summary

• What: Adds if: github.repository == 'PecanProject/sipnet' to the link-check job in .github/workflows/link-check.yml.
• Motivation: Prevents the weekly scheduled link-check cron job from running unnecessarily on contributor forks and generating spurious failure notifications.

How was this change tested?

• Verified YAML syntax and job configuration.
• pull_request runs targeting PecanProject/sipnet will evaluate to true and continue running normally.

Related issues

• Fixes Scheduled link-check workflow runs on contributor forks with no changes #373

Checklist

• Related issues are listed above.
• PR title has the issue number in it ([#] <concise description of proposed change>).
• Tests added/updated for new features (not applicable for workflow configuration).
• Documentation updated (not applicable).
• Code formatted with clang-format.

[Copilot]

Pull request overview

Adds a repository guard to the scheduled “Broken Link Check” GitHub Actions workflow so it only runs in the canonical upstream repository (PecanProject/sipnet), preventing scheduled executions on contributor forks.

Changes:

• Added if: github.repository == 'PecanProject/sipnet' at the link-check job level in .github/workflows/link-check.yml to skip the job on forks.

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[Alomir]

Looks good, thanks!