Fix tests for Windows ARM64 and add to CI

[andrjohns]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

The tests needed a minor change for compatibility with Windows ARM64 - installing cmdstan versions which weren't compatible. This PR fixes the tests and adds the platform to CI

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.08%. Comparing base (c7b188d) to head (6fcde41).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1201      +/-   ##
==========================================
- Coverage   91.10%   91.08%   -0.02%     
==========================================
  Files          15       15              
  Lines        6127     6127              
==========================================
- Hits         5582     5581       -1     
- Misses        545      546       +1     

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[jgabry]

Great, thanks!