postCSS mixins with nested selectors ignored

[saltnpixels]

postCSS mixins with nested selectors do not seem to get compiled and are ignored.

Plugins in use:
https://github.com/csstools/postcss-nesting
https://github.com/postcss/postcss-mixins

[Antonio-Laguna]

This can be solved by making sure this plugin runs first. It probably happens due to the plugin being stuck at PostCSS 7 but this is a sensible workaround for the time being.