refactor: use path.sep

Co-authored-by: Bjorn Lu <[email protected]>

Author: sapphi-red

packages/vite/src/node/config.ts

@@ -1721,7 +1721,7 @@ async function bundleConfigFile(
     mainFields: ['main'],
     sourcemap: 'inline',
     // the last slash is needed to make the path correct
-    sourceRoot: path.dirname(fileName) + '/',
+    sourceRoot: path.dirname(fileName) + path.sep,
     metafile: true,
     define: {
       __dirname: dirnameVarName,