WIP: CI test windows-2025-vs2026

[dzenanz]

DO NOT MERGE: testing windows-2025-vs2026 OS. Background.

[dzenanz]

This fails with a strange error:

CMake Error at .pixi/envs/cxx/Library/share/cmake-4.1/Modules/CMakeDetermineCXXCompiler.cmake:47 (message):
  Could not find compiler set in environment variable CXX:

  cl.exe.

Call Stack (most recent call first):
  CMakeLists.txt:59 (project)

CMake Error: CMAKE_CXX_COMPILER not set, after EnableLanguage
CMake Error: CMAKE_C_COMPILER not set, after EnableLanguage
-- Configuring incomplete, errors occurred!
Error: Process completed with exit code 1.

@hjmjohnson There is also this surprising warning: WARN the lock file is up-to-date but uses an older format (v6), run pixi lock to upgrade to v7 for improved reproducibility.

[hjmjohnson]

@dzenanz The lock-file warning is independent of this PR.

• pixi.lock line 1 on main is version: 6, written by a pre-pixi-0.50 binary. Pixi ≥ 0.50.0 (released 2025-07-24, ~9.5 months ago) prefers version: 7 and emits the warning when it reads v6. The build still works correctly — it's informational, not blocking.
• ENH: Use latest version (v0.9.5) of setup-pixi GitHub action #6262 bumps prefix-dev/setup-pixi@v0.8.1 → v0.9.5. That changes how pixi is installed on the runner; neither action version pins pixi-version:, so both install the latest stable pixi at run time. The lock file itself is never touched by the action, so the warning persists.
• Fix is a one-file PR: run pixi lock locally and commit the regenerated pixi.lock. Pixi re-solves using the existing locked content — no package version changes, only the format / ordering. Opened as COMP: Upgrade pixi.lock from v6 to v7 format #6264.

[dzenanz]

Abandoning investigation why cl.exe was not found. Not yet urgent.