test out windows-11-vs2026-arm

[jjhelmus]

testing out effect of switching to VS2026 in the aarch64 windows runner for the upcoming migration:

https://github.blog/changelog/2026-08-20-windows-11-arm64-vs2026-image-generally-available/

[vadikmironov]

@jjhelmus, I ran your question through a matrix on windows-11-vs2026-arm while working on #1262.

Here is the summary of the findings if it helps:

• --vs 2022 cannot be used on new image at all: find_vs_path asks vswhere for a VS 17 install and there is none.
• With --vs 2026, only 3.15 builds. 3.11 to 3.14 all fail with MSB8020: 3.13 and 3.14 map VisualStudioVersion 18.0 to v143 in PCbuild/python.props, 3.11 and 3.12 have no mapping and fall through to v140: link to failed run
• The v143 failure is not controlled by vcvars. pcbuild.proj builds _freeze_module as Win32 for an ARM64 target, and the arm64 image ships v143 for the ARM64 target only (the x64 image ships it for x86/x64 only). v145 is the only toolset both images carry for every host and target.
• Passing /property:PlatformToolset=v145 for 3.10 to 3.14 under --vs 2026 makes all 16 green, pgo cycle included: successful run. The change is 16 lines in build.py: commit link

Happy to open it as a PR, or you can fold it into this one, whichever you prefer.