Rtools43 support

[andrjohns]

Submission Checklist

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

Summary

There was a report on the forum that cmdstanr's toolchain checks weren't working with RTools43, this PR updates the functions to correctly detect the RTools42 vs RTools43

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):
Andrew Johnson

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]

Awesome. Assuming tests pass on the rtools43 GHA instance, feel free to merge. Thanks!

[codecov-commenter]

Codecov Report

Merging #755 (1fdd923) into master (a30d3e9) will decrease coverage by 2.30%.
The diff coverage is 26.66%.

❗ Current head 1fdd923 differs from pull request most recent head 94f6395. Consider uploading reports for the commit 94f6395 to get more accurate results

@@            Coverage Diff             @@
##           master     #755      +/-   ##
==========================================
- Coverage   88.36%   86.07%   -2.30%     
==========================================
  Files          12       12              
  Lines        4099     4107       +8     
==========================================
- Hits         3622     3535      -87     
- Misses        477      572      +95     

Impacted Files	Coverage Δ
R/install.R	49.22% <15.38%> (-13.68%)	⬇️
R/utils.R	72.64% <100.00%> (-1.87%)	⬇️

... and 3 files with indirect coverage changes

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