Build: Fix demo asset copying

- Don't copy SCSS files to unmin
- Use better scope for copying umnin assets to minfied

Fixes gh-3867
Build: Fix CSS minification targets

Previously the CSS was copied from unmin and left an extra unminified
copy after minification.
Merge pull request #3871 from nschonni/v4-fix-demo-asset-copying

Build: fix demo asset copying and minification