fix(css): root relative import in sass modern API on Windows

[sapphi-red]

Description

fixes #18919

[pkg-pr-new]

Open in Stackblitz

npm i https://pkg.pr.new/vite@18945

commit: edbe5cd

[sapphi-red]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI on edbe5cd: Open

suite	result	latest scheduled
histoire	❌ failure	❌ failure
laravel	❌ failure	✅ success
marko	❌ failure	✅ success
redwoodjs	❌ failure	❌ failure
remix	❌ failure	❌ failure
vike	❌ failure	❌ failure
vite-plugin-svelte	❌ failure	❌ failure
waku	❌ failure	❌ failure

✅ analogjs, astro, ladle, nuxt, previewjs, quasar, qwik, rakkas, storybook, sveltekit, unocss, vite-environment-examples, vite-plugin-pwa, vite-plugin-react, vite-plugin-react-swc, vite-plugin-vue, vite-setup-catalogue, vitepress, vitest, vuepress