Consolidate hashFiles() calls in the build.yml workflow

[mcdonnnj]

🗣 Description

This pull request updates the cache key we use in the build.yml workflow to use a single call to hashFiles() with all files instead of individual calls per file.

💭 Motivation and context

This generate a single derived hash from the files which will reduce the length of the cache key used. This is mostly just to ease interaction with the gh cache extension and when inspecting the log for a workflow run.

🧪 Testing

Automated tests pass.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All new and existing tests pass.

[Copilot]

Pull request overview

This PR streamlines the GitHub Actions cache key used in .github/workflows/build.yml by consolidating multiple hashFiles() invocations into a single call that hashes all relevant dependency/config files together, reducing cache key length for easier inspection and interaction with tooling like gh cache.

Changes:

• Replace three separate hashFiles() calls with a single hashFiles(pattern1, pattern2, pattern3) call.
• Shorten/simplify the resulting cache key while keeping it sensitive to the same set of input files.

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