turbopack-node: Use path.join for postcss loader (#8105)
### Description
A followup requested by @sokra in
https://github.com/vercel/turbo/pull/7995#discussion_r1568285539
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>
</details>
Closes PACK-3049
