Add HUGO_PUBLISHDIR to the Node environment

Add HUGO_PUBLISHDIR to the Node environment

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