Use Windows ARM64 for RTools CI

[andrjohns]

Summary

Windows ARM runners are now available for public repos, so we can add them to the CI. I've just replaced the existing windows runners, but can also add them separately if we want both x86-64 & arm64 rtools.

The setup-r action doesn't support arm64, so I've just added a step to manually install RTools

Tests

N/A - existing tests should still pass

Side Effects

N/A

Release notes

Added Windows ARM64 to the Github CI

Checklist

• 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

[WardBrian]

There are several orders of magnitude more x86 windows users than ARM, so if we do test this at all we should do it in addition rather than as a replacement

[andrjohns]

There are several orders of magnitude more x86 windows users than ARM, so if we do test this at all we should do it in addition rather than as a replacement

Ah yeah true, will add both

[WardBrian]

It looks like sometimes these jobs get stuck installing rtools, e.g: https://github.com/stan-dev/math/actions/runs/14649317253/job/41111049475?pr=3171

1 hour and counting just in the install step

[WardBrian]

I can't tell if it's a rate limiting issue or the installer thats hanging. If its the former we could add an authorization header to Invoke-WebRequest