How to add an ibus engine into to NixOS

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I would say the issue is that the derivation does not actually install any files:

$ nix-build -I "nixpkgs=channel:nixos-unstable" -E 'with import <nixpkgs> {}; callPackage ./ibus-bamboo.nix {}'
/nix/store/xhp6zfsmc1h407jk6xx6gn7wci7lwdns-ibus-bamboo-v0.6.5
$ exa -T result/
result

Not really familiar with Go packaging but perhaps you need buildGoModule or buildGoPackage.