* nix-channel: fail if we don't have write permission to the manifests   directory.  Previously in this situation we did add the Nix   expressions from the channel to allow installation from source, but   this doesn't work for binary-only channels and leads to confusing   error messages.
* nix-channel: fail if we don't have write permission to the manifests
  directory.  Previously in this situation we did add the Nix
  expressions from the channel to allow installation from source, but
  this doesn't work for binary-only channels and leads to confusing
  error messages.


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