Make use of untar() platform independent in install_cmdstan()

[jgabry]

Submission Checklist

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

Summary

Fixes #1029
Uses @WardBrian suggestion of switching untar's exdir to point to the parent directory, which would allow us to

drop the --strip-components flag entirely, and then it wouldn't matter which tar implementation gets used, including internal. That way the testing/verifying the change is platform independent

Copyright and Licensing

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Columbia University

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license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
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[WardBrian]

@jgabry not sure if you wanted a formal "review" or not since this is so small, but I can confirm this is exactly the change I was imagining

[jgabry]

Thanks. I think this is good to go then. I don't think we've had anyone test this on older Windows (@yizhang-yiz tested it for me on Windows 11), but it sounds like separately testing on Windows 10 shouldn't be necessary. Or do you think we should find someone running Windows 10 to be extra safe?

[WardBrian]

Should be fine I think -- if this doesn't work, the bug would be in R's untar, since you're not passing anything bespoke any longer