Add Python Version Missing from actions/cache Key

[mcdonnnj]

🗣 Description

This PR adds the installed Python version as an element of the cache key used in the test job of the build workflow.

💭 Motivation and Context

When I was looking over #39 I noticed that the test job did not have the Python version in its actions/cache key. This is an oversight that slipped through, but needs to be fixed.

🧪 Testing

Automated testing passes.

✅ 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.

[jsf9k]

Great minds think alike!

[dav3r]

Good catch! 👍