Detect OS more consistently

[enkessler]

This should fix #6720 and implements the suggested solution.

I did not add new tests, as testing this issue would require adding additional matrix combinations to the AppVeyor build (there is no JRuby/Windows combo being tested in CI) and I have no desire at this time to debug any failures on an entirely new build combination. I did run the default rake task locally and the results are no worse after than before the changes in this PR.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[Drenmi]

Thank you! 🙇‍♂️

[koic]

Thanks!