Add a delimiter between two hashes

[jsf9k]

🗣 Description

This pull request adds a - delimiter between two hashes.

💭 Motivation and context

Previously the hash for requirements.txt was concatenated directly with the hash for pyproject.toml. This can increase the chance of ambiguous cache keys and makes the key harder to read/compare with the other jobs (which use a - separator).

🧪 Testing

All 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 makes the test-source workflow cache key consistent with the other jobs by inserting a - separator between the requirements.txt hash and the pyproject.toml hash. In the broader workflow configuration, this reduces cache-key ambiguity and aligns formatting across Python job caches.

Changes:

• Add a missing - delimiter in the test-source job’s actions/cache key.
• Align the test-source cache key structure with the existing cache key patterns used elsewhere in build.yml.

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