Feature flags: cache does not obey READ operation

```py
@with_repository(fake='dry_run', exclusive=True, compatibility=(Manifest.Operation.WRITE,))
def do_create(self, args, repository, manifest=None, key=None):
```

But the cache may be synced, which needs Operation.READ, which is not checked.

The cache sync thus needs to perform another operation check with READ, independent of what the command opening the cache may have checked.