fix Windows torch source marker

[zhonggang666]

What this PR does / why we need it?

Fixes the Windows torch uv source marker.

Closes #6368.

Summary of your change

Changes the Windows marker from sys_platform == 'win' to sys_platform == 'win32', which is the value used by Python packaging markers on Windows.

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

Validation:

• git diff --check
• packaging.markers.Marker("sys_platform == 'win32'").evaluate({'sys_platform': 'win32'}) returns True.