Use utils::capture.output to fix r cmd check note

[jgabry]

Submission Checklist

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

Summary

Fixes r cmd check note

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.64%. Comparing base (8f15e30) to head (b1fba36).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1161   +/-   ##
=======================================
  Coverage   90.64%   90.64%           
=======================================
  Files          14       14           
  Lines        5934     5934           
=======================================
  Hits         5379     5379           
  Misses        555      555           

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