Update to rtools43 in CI

[andrjohns]

Summary

Our GHA CI for running Windows headers/tests is using RTools40. Given that the current R version is 4.3, we should be testing using RTools43.

This PR updates our CI to use RTools43, following the installation instructions in the manual: https://mc-stan.org/docs/cmdstan-guide/cmdstan-installation.html#rtools42-rtools43

Tests

N/A - current tests should still pass

Side Effects

N/A

Release notes

Update CI to use RTools43

Checklist

• Math issue #(issue number)

• Copyright holder: Andrew Johnson

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the 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/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[andrjohns]

Closing for some debugging