Improve logic for comparing cmdstan installation versions

[jgabry]

Submission Checklist

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

Summary

Ensure that comparison of cmdstan versions always can identify the most recent. We never hit this bug in practice but in theory 2.9 could have been flagged as more recent than 2.39.

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):
Jonah Gabry

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

❌ Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.24%. Comparing base (f2fd044) to head (0da85aa).

Files with missing lines	Patch %	Lines
R/path.R	96.00%	1 Missing ⚠️
R/run.R	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1193      +/-   ##
==========================================
+ Coverage   91.23%   91.24%   +0.01%     
==========================================
  Files          15       15              
  Lines        6124     6145      +21     
==========================================
+ Hits         5587     5607      +20     
- Misses        537      538       +1     

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