fix: remote.Referrers: Set remoteIndex.ref to resolve panics in Index manifests

[bmoylan]

When remote.Referrers() returns a v1.Index, fetching derivative objects (like referenced manifests) requires keeping the Reference to the original repository.

Without setting the ref field, we get nil pointer dereference panics in (*remoteIndex).childDescriptor()

go-containerregistry/pkg/v1/remote/index.go

Lines 206 to 207 in 005bb71

	func (r *remoteIndex) childDescriptor(child v1.Descriptor, platform v1.Platform) (*Descriptor, error) {
	ref := r.ref.Context().Digest(child.Digest.String())

[bmoylan]

Hi @jonjohnsonjr @imjasonh hoping you can take a look at this panic fix. I hope I'm not missing any side effects with the *remote.Index now holding a ref that doesn't actually exist in the remote's manifest store (since the referrers index is created on-the-fly)