oci.pull is too sensitive to credential helper failures

[thesayyn]

Currently, if oci.pull fails when fetching credentials from credhelpers if the docker/config.json has a corresponding entry in auths dictionary.

EG:

{
  "auths": {
      "ghcr.io": {}
  },
  "credsStore": "desktop"
}

Even if oci.pull is pulling a public image, it tries to fetch credentials which will lead to failures since the credential helper has no credentials stored.

rules_oci/oci/private/authn.bzl

Line 236 in d884f58

pattern = _fetch_auth_via_creds_helper(rctx, host_raw, config["credsStore"])