oras cp showing wrong digest

[qweeah]

When copying an image of a certain arch from an index using --platform, if

• recursively copying (-r is used), or
• no tag specified in the target registry

Then the digest of index will be shown in the copy summary.

ORAS version

Version:        1.0.0-rc.1
Go version:     go1.19.5
Git commit:     a98931bc68a0e7a5de6c51df8e5aa09aadad3057
Git tree state: clean

Reproducing steps

# prepare index
oras cp ghcr.io/oras-project/oras:v1.0.0-rc.1 --to-oci-layout ./local/oras:v1.0.0-rc.1 -v

# attach to linux/s390x image
echo hello > hello
oras attach --artifact-type test.hello --oci-layout ./local/oras@sha256:d1903141acefdeed52e34811d495f7569aba7b97d69c5a7ad22a3f213679b117

# showing wrong digest when recursively copying
 oras cp --from-oci-layout ./local/oras:v1.0.0-rc.1 --to-oci-layout ./local/oras:v1.0.0-rc.1-test --platform linux/s390x -v
-r

# showing wrong digest when copying without tagging
 oras cp --from-oci-layout ./local/oras:v1.0.0-rc.1 --to-oci-layout ./local/oras --platform linux/s390x -v -r

Expected behavior

The image digest of the chosen arch should be shown.

This bug effects all released versions after 0.15.0

[qweeah]

Fixed by #818