use setenv to avoid parallel-processx-conflict

[avehtari]

Fixes #1105

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):
Aki Vehtari

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 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.21%. Comparing base (5f2a8c8) to head (0032564).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
R/zzz.R	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1106      +/-   ##
==========================================
- Coverage   87.22%   87.21%   -0.02%     
==========================================
  Files          14       14              
  Lines        5973     5974       +1     
==========================================
  Hits         5210     5210              
- Misses        763      764       +1     

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[jgabry]

Looks good, thanks @avehtari!