turbopack-node: Use path.join for postcss loader (vercel/turborepo#8105)

### Description

A followup requested by @sokra in
vercel/turborepo#7995 (comment)

There's no meaningful change in behavior here, as node on windows
interprets both `/` and `\` as valid path separators, but this makes the
intent clearer.

### Testing Instructions

<details><summary>Repeated the test plan for #7995. Tested on both
Windows and Linux.</summary>

![Screenshot 2024-05-07 at
2.22.17 PM.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/HAZVitxRNnZz8QMiPn4a/bf7afcfb-0834-454a-b5ee-f52ae736eb47.png)
</details>


Closes PACK-3049

crates/turbopack-node/src/transforms/postcss.rs

@@ -248,9 +248,10 @@ pub(crate) async fn config_loader_source(
     // Bundling would break the ability to use `require.resolve` in the config file.
     let code = formatdoc! {
         r#"
-            import {{ pathToFileURL }} from 'url';
+            import {{ pathToFileURL }} from 'node:url';
+            import path from 'node:path';
 
-            const configPath = `${{process.cwd()}}/${{{config_path}}}`;
+            const configPath = path.join(process.cwd(), {config_path});
             // Absolute paths don't work with ESM imports on Windows:
             // https://github.com/nodejs/node/issues/31710
             // convert it to a file:// URL, which works on all platforms