Fix .github/workflows/backend.yml

[Matthew-Grayson]

🗣 Description

Fix .github/workflows/backend.yml by adding single quotes around file path on line 117.
This fix matches the formatting used for the hashFiles() method across the code base.

💭 Motivation and context

PR #32 broke this file (see screenshot below).

🧪 Testing

📷 Screenshots

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced
  in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Create a release.

[dav3r]

Approved with same caveat as #32:

@Matthew-Grayson stated that the remaining automated test failures will be addressed in future PRs.