streamLayeredImage: pass through passthru attributes
streamLayeredImage: pass through passthru attributes

This is useful for a use-case we have with a Nix-based CI system that
specifies things like deploy steps as passthru attributes[0].

Previously the only way to do this would have been to concatenate
attributes onto the resulting derivation, but passing them in and
actually treating them as proper passthru attributes is cleaner.

[0]: https://cs.tvl.fyi/depot@f7d7da6aceb407b719cf4683a75878fd3aca319e/-/blob/nix/buildkite/default.nix?L222-226

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