Unable to validate code scanning workflow

## Summary

When running scans the Security Code Scanner gives the following warning:

Example: https://github.com/MetaMask/metamask-extension/actions/runs/17072938137/job/48406519510#annotation:2:472

> [MetaMask Security Code Scanner](https://github.com/MetaMask/metamask-extension/actions/runs/17072938137/job/48406519510#annotation:2:472)
Unable to validate code scanning workflow: error: getWorkflow() failed: Error: Expected to find a code scanning workflow file at /home/runner/work/metamask-extension/metamask-extension/.github/workflows/security-code-scanner.yml, but no such file existed. This can happen if the currently running workflow checks out a branch that doesn't contain the corresponding workflow file.

Start with the referenced .github/workflows/security-code-scanner.yml path and the linked GitHub Actions run to determine why the scanner cannot find the workflow in the checked-out branch. Read the action's workflow-validation entry point and reproduce the warning with a representative workflow; done means the scanner validates the intended workflow without this error.

Written by the indexing model from the issue text.