E.g. when pushing a schema2 image, dockerImageDestination.TryReusingBlobWithOptions can find and choose to use a Zstd layer

• That directly breaks if the user forced schema1/schema2 manifest type
• Even without copy.Options.ForceManifestMIMEType, if the destination is manifest-schema-agnostic, and determineManifestConversion has no reason to expect Zstd to be involved, the copy code will only have a single candidate; we need to either add an OCI manifest schema candidate, or not reuse Zstd layers if the copy is not set up to use OCI.