ci: Fix pre-generating stylesheets in dist tarball
Oh boy, that went quite wrong:
- the gnome-shell-sass submodule isn't inited in CI, so its content is missing
- the generate-stylesheets.py script doesn't fail on sassc errors, so the dist job still completed successfully
- the (fedora-)build job didn't specify the correct classic_mode option, so it didn't catch the issue either