MacOS rustPackages build broken because of libiconv

[Profpatsch]

When updating to 21.05, an upstream bug broke the Mac build. Should be fixed before the next release:

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Okay, apparently this won’t work for now because of https://github.com/NixOS/nixpkgs/pull/122231.

Merging and hoping it will solve itself before the next release.

Originally posted by @Profpatsch in https://github.com/nix-community/lorri/issues/51#issuecomment-864890216

[sternenseemann]

May be related: https://github.com/NixOS/nixpkgs/issues/126182