Switch registry tag for the stable branch.

At first the librsvg registry was using {repo}/{distro}:{version} scheme for tagging the images.

Since the i386 builds were added and with the arm coming in the future to I switched it {repo}/{ARCH}/{distro}:{version}.

The old images are there too but they won't be getting updated. It would also be nice to clean them and have the oci-registry a bit more organized.