[unplugin] Fix the path check for Windows

[siriwatknp]

Issue

On Windows, the received id is ...\@pigment-css\react\theme but the unplugin only check for /. That's why the generateThemeSource() is not called which is result in {} for runtime theme and the build is error for components that try to use the theme.

Before: https://github.com/mui/material-ui/actions/runs/9475564524/job/26107055930?pr=42476

After: https://github.com/mui/material-ui/actions/runs/9476406034/job/26109235879?pr=42476

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