cleanly handle compiling with threads in compile doc

[jgabry]

Submission Checklist

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

Summary

The $compile() example recompiles the CmdStan example model with stan_threads = TRUE. Because pkgdown before building vignettes, the cmdstanr.Rmd vignette then reused that threaded executable and errored when trying to run sampling.

This was always happening, but before #1212 the threading metadata went undetected due to the case-sensitivity bug.

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 91.87%. Comparing base (4a3ddb8) to head (839ede6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1222      +/-   ##
==========================================
+ Coverage   91.79%   91.87%   +0.08%     
==========================================
  Files          15       15              
  Lines        6249     6241       -8     
==========================================
- Hits         5736     5734       -2     
+ Misses        513      507       -6     

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