Fix use of tilde with cmdstanr_write_stan_file_dir

[rok-cesnovar]

Submission Checklist

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

Summary

See https://discourse.mc-stan.org/t/problem-using-cmdstanr-write-stan-file-dir-with-brms/28117 for reasons
Essentially using ~ with the cmdstanr_write_stan_file_dir option did not work. This fixes that.

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):
Rok Češnovar

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/)

[rok-cesnovar]

@sims1253 confirmed the fix works for the reported issue so I am going to merge once the tests pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.77%. Comparing base (953fdd0) to head (414843b).
⚠️ Report is 902 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #665   +/-   ##
=======================================
  Coverage   89.77%   89.77%           
=======================================
  Files          12       12           
  Lines        3529     3530    +1     
=======================================
+ Hits         3168     3169    +1     
  Misses        361      361           

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