fix: use pixi-compatible variable syntax in Windows tasks

[miyanyan]

What

This updates the Windows pixi task/env definitions in:

• cuda_pathfinder/pixi.toml
• cuda_bindings/pixi.toml
• cuda_core/pixi.toml

It replaces cmd.exe-style %VAR% references with $VAR syntax in Windows-specific pixi task and environment entries.

Why

pixi run executes tasks via deno_task_shell, not cmd.exe or PowerShell.

Because of that, values like %PIXI_PROJECT_ROOT% are passed through literally instead of being expanded on Windows. That causes commands to fail with errors like:

PS D:\github\miyanyan\cuda-python\cuda_pathfinder> pixi run test
Pixi task (test in default): pytest %PIXI_PROJECT_ROOT%                                                                                          
============================================================= test session starts ============================================================== 
platform win32 -- Python 3.14.2, pytest-9.0.2, pluggy-1.6.0
Using --randomly-seed=822857797
rootdir: D:\github\miyanyan\cuda-python\cuda_pathfinder
configfile: pyproject.toml
plugins: mock-3.15.1, randomly-3.15.0, repeat-0.9.4
collected 0 items                                                                                                                                

============================================================ no tests ran in 0.02s ============================================================= 
ERROR: file or directory not found: %PIXI_PROJECT_ROOT%

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