CI: in distcheck, also save the .sha256sum artifact
I'm not using it right now, since "ftpadmin install" on master.gnome.org creates its own checksum, but it seems odd not to have it since Meson creates it when it creates the release tarball.
I'm not using it right now, since "ftpadmin install" on master.gnome.org creates its own checksum, but it seems odd not to have it since Meson creates it when it creates the release tarball.