cherry-pick packages in nix between channels

cherryPickOverlay.nix

# Motivation:
#  - System uses <nixos> 20.09
#  - Pulling any package from <nixpkgs-unstable> effectively keeps part of the system duplicated

self: super:
let
  nixpkgs-unstable = import <nixpkgs-unstable> { };
  cherryPick = pkg: pkg.override (origArgs: builtins.intersectAttrs origArgs self);
in {
  zig-unstable = (cherryPick nixpkgs-unstable.zig).override {
    llvmPackages = self.llvmPackages_11;
  };
}