Use Windows ARM64 hosted runners

[hugovk]

These are now in "public preview" for public repos:

https://github.blog/changelog/2025-04-14-windows-arm64-hosted-runners-now-available-in-public-preview/

To use them, we use runs-on: windows-11-arm instead of windows-latest.

This means we can test ARM64 for both upstream and forks, and simplify the CI config.

[hugovk]

This new windows-11-arm runner is a standard sized one with 4 CPUs, and the old windows-aarch64 is a larger runner with 8 CPUs.

Time comparison:

job	windows-11-arm	windows-aarch
Regular test	15m15s	11m43s
Free-threaded test	21m54s	14m16s
JIT debug	19m02s	13m55s
JIT release	17m54s	13m31s

This is similar to the other Windows jobs, which take around ~18m-24m.

There are larger images available, but it's unclear if these are paid:

In partnership with Arm, there is now a Windows 11 desktop arm64 image with preinstalled tools available for all GitHub runner sizes, including both the new free offering and our existing arm64 larger runners. To use the new image on larger runners, you can create a new runner and select the Microsoft Windows 11 Desktop by Arm Limited image in the Images console.

https://github.blog/changelog/2025-04-14-windows-arm64-hosted-runners-now-available-in-public-preview/#images-for-arm64-larger-runners

[ewdurbin]

The current spend rate on ARM is astonishing. The performance penalty is unfortunate, but moving to the free runners is definitely the right move.

[brandtbucher]

JIT workflow looks good.