So you can do `process.env.HUGO_PUBLISHDIR` in your `postcss.config.js` to figure out where Hugo publishes
its files.
Note that the value will always be an absolute file path and will point to a directory on disk even when running `hugo server` in memory mode.
If you write to this folder from PostCSS when running the server, you could run the server with one of these flags:
```
hugo server --renderToDisk
hugo server --renderStaticToDisk
```
Fixes#10554
