- Do you want to request a feature or report a bug?

It's a feature 🙏 , not a bug.

- What is the current behavior?

From version 0.43 Hugo comes in two flavours, extended version allows native Sass compilation.
Netlify currently does not support extended version:

2:28:30 PM: ERROR 2018/07/10 12:28:30 error: failed to transform resource: failed to transform “scss/main.scss” (text/x-scss): this feature is not available in your current Hugo version

- What is the expected behavior?

Either download the extended binary by default from now on or allow to specify something like 0.43/extended in netlify.toml.

_extended is added to the release path before the version number:

https://github.com/gohugoio/hugo/releases/download/v0.43/hugo_0.43_Linux-64bit.tar.gz
https://github.com/gohugoio/hugo/releases/download/v0.43/hugo_extended_0.43_Linux-64bit.tar.gz