Incorrect postcssOpts.to passed to postcss plugins

While investigating an issue with path transforms, it seems I might have hit a bug in this plugin. I created a example to reproduce the issue with minimal settings here: https://github.com/osdevisnot/rollup-plugin-postcss-demo

In a nutshell, it seems we are passing incorrect to option to postcss plugins which causes issues with path transformation for `postcss-copy-assets`

Let me know if I should add more information to this bug report, but the [README here](https://github.com/osdevisnot/rollup-plugin-postcss-demo/blob/master/README.md) summarizes issue correctly: