Separate gvisor_download_url for runsc and shim (#7760)

To download necessary files in advance for offline deployment,
we can see all file URLs with contrib/offline/generate_list.sh
Most URLs are downloadable, but gvisor's one is not because the
URL is a part of full URLs for gvisor.
To download gvisor's files from the URLs directory, this separates
into two URLs for runsc and the shim.