Allow git:TAG as a version in install_cmdstan

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

Allows usage like install_cmdstan --version=git:develop. Note: released versions will still always be selected over git versions by the cmdstan_path() function unless manually set or only one version exists.

This is primarily for our own 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):

Simons Foundation

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)

[mitzimorris]

LGTM and runs on my machine!

[codecov-commenter]

Codecov Report

Merging #679 (df5c3a4) into develop (9b01be8) will decrease coverage by 0.22%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #679      +/-   ##
===========================================
- Coverage    81.01%   80.79%   -0.22%     
===========================================
  Files           69       69              
  Lines        10677    10734      +57     
===========================================
+ Hits          8650     8673      +23     
- Misses        2027     2061      +34     

see 6 files with indirect coverage changes

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