CI: do not install gi-docgen on non-opensuse distros
CI: do not install gi-docgen on non-opensuse distros

It's not used there.

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/754>

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg