bug: `manifest fetch` fails when confirming the platform of a manifest

# Reproduce step
```
oras manifest fetch ghcr.io/oras-project/oras@sha256:ec82a074d971186e5c47080b7eb35275e61f7c3be8303d1f891923d0109f239f --platform linux/arm64
# Error: sha256:bd06290a13e684209732bcfd5747f617b601e1d5fef1db0c07266546dd0941e7: not found
```

Root cause is that the a manifestStore is used for blob(config) fetching