Fix untar() on Windows 11

[jgabry]

Closes #1029

Submission Checklist

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

Summary

Uses @svethitov's suggestion in https://github.com/stan-dev/cmdstanr/issues/1029#issuecomment-2466214417 of manually specifying tar.exe on Windows 11 to avoid issue with Sys.getenv("TAR").

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): Columbia University

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/)

[jgabry]

Holding off on this for now. It might be superseded by https://github.com/stan-dev/cmdstanr/pull/1034

[jgabry]

Closing in favor of the approach in #1034