nix: Add libiconv as build input On some mac devises one cannot depend on libiconv being resolved by the linker and thus compilation of Pijul fails. This change simply adds the dependency to the list of build inputs and as such allows the project to compile again.

Created by  zj  on August 13, 2021
FRO3FIIVKLJXZLGCB4EPNTHLZKB5IKIZ5FFOKOGJAFSE7PEEP4LQC
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