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zipline: set NODE_PATH for sharp
When building sharp from source, it tries to `require("node-gyp")`,
which will not be found since it's not in zipline's deps. We therefore
set NODE_PATH with nixpkgs's node-gyp, so that it can be imported by the
sharp build script.
See:
https://github.com/lovell/sharp/blame/7b4c4762432b14c62676e860c8034b5cd326f464/install/build.js#L24