bug: The flag --to-distribution-spec does not work with oras copy

[Wwwsylvia]

What happened in your environment?

When using the flag --to-distribution-spec v1.1-referrers-tag with oras copy, I expect a referrers tag to be created in my target registry. However, the flag does not take effect and the referrers tag is not created.

What did you expect to happen?

Referrers tag schema is enforced, and a referrers tag is created in the target registry.

How can we reproduce it?

oras copy -r $srcRegistry $dstRegistry --to-distribution-spec v1.1-referrers-tag

What is the version of your ORAS CLI?

Version: 0.16.0+unreleased
Go version: go1.19.3
Git commit: 6e876c1

What is your OS environment?

Windows

Are you willing to submit PRs to fix it?

• Yes, I am willing to fix it.