fix: add fallback for ref parameter in cache-checkout action

[keithwillcode]

What does this PR do?

Addresses feedback from cubic's review on PR #26702.

The cache-checkout action's fallback checkout step was missing a fallback for the ref parameter, which would break the action in non-PR contexts (e.g., workflow_dispatch). The cache key already correctly uses ${{ github.event.pull_request.head.sha || github.sha }} but the checkout ref lacked the same fallback.

This PR adds the || github.sha fallback to match the cache key pattern.

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A - CI workflow changes only.
• I confirm automated tests are in place that prove my fix is effective or that my feature works. N/A - CI workflow changes can only be tested by running the actual CI pipeline.

How should this be tested?

1. Trigger a workflow that uses cache-checkout via workflow_dispatch (non-PR context)
2. Verify the checkout step succeeds using github.sha as the fallback ref

Checklist

• My code follows the style guidelines of this project
• I have checked if my changes generate no new warnings

Human Review Checklist

• Verify the fallback || github.sha is appropriate for all workflow contexts where this action is used
• Confirm the pattern matches the cache key on line 15

[devin-ai-integration]

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[cubic-dev-ai]

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