Correctly fetch ppc64el CmdStan

[WardBrian]

Submission Checklist

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

Summary

Resolves #660

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)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.56%. Comparing base (aae75b4) to head (261493a).
⚠️ Report is 278 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #661      +/-   ##
===========================================
+ Coverage    80.54%   80.56%   +0.01%     
===========================================
  Files           69       69              
  Lines        10641    10641              
===========================================
+ Hits          8571     8573       +2     
+ Misses        2070     2068       -2     

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