R: fix build on Windows ARM64

[jeroen]

See failure on https://github.com/r-universe/apache/actions/runs/31226347531/job/93050529031

More information: https://contributor.r-project.org/windows-arm64/

[james-willis]

@paleolimbot do we want to add any kind of CI target for this?

[yutannihilation]

Looks great!

any kind of CI target for this

As r-lib/actions already supports Windows ARM64, it should be easy to add ARM64 Windows runner on GitHub Actions.

e.g. https://github.com/yutannihilation/savvy/pull/457/changes#diff-9c940e8ad2b7bc4c26ec3da57b94bc00e73e2166cfed689da51a4c59bcc0a310

But, considering r.yml runs only one case for each OS (e.g. no Intel macOS), I think we don't want to add it in order to keep the CI setup simple.