update which skynet package is used

currently, https://github.com/containerish/OpenRegistry/blob/c7bc9ba85b92ad1c4b59f5b50caf2a1fe8bac967/skynet/skynet.go#L9 imports from https://github.com/NebulousLabs/go-skynet which appears to be archived an directs users to use https://github.com/SkynetLabs/go-skynet instead