Separate Coveralls build results by platform

[mcdonnnj]

🗣 Description

This pull request updates the flag used for Coveralls test results to include the platform and not just the Python version.

💭 Motivation and context

The description for using Coveralls in parallel such as we do indicated that the flag-name variable must be unique. Since we have started multi-platform testing that has not been the case and it appears to be causing problems in getting Coveralls test results reported. This appears to rectify the issue.

🧪 Testing

Automated tests pass. I verified that Coveralls appears to be processing the test results whereas before it seemed unable to process the uploaded files (likely because of namespace collision).

✅ 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.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.

[Copilot]

Pull Request Overview

This pull request updates the Coveralls integration to use unique flag names that include both Python version and platform information, addressing issues with parallel test result processing across multiple platforms.

• Modified the flag-name parameter in the Coveralls GitHub Action to include platform information alongside Python version
• This ensures unique identifiers for each test run combination, preventing namespace collisions in Coveralls reporting

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