CI: Install google-roboto-fonts for rsvg-convert's tests
CI: Install google-roboto-fonts for rsvg-convert's tests

The container images didn't have any system fonts.  Without any fonts,
PDFs with text content don't get generated correctly.

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/673>

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