nix: create overlay and use it throughout the repo

nix/home-manager/profiles/common.nix

@@ -25,9 +25,6 @@ in {
       dropbearStatic = dropbear.override {
         enableStatic = true;
       };
-
-      just = pkgs.callPackage ../../pkgs/just.nix {};
-      duplicacy = pkgs.callPackage ../../pkgs/duplicacy {};
     };
   };
 

nix/os/devices/steveej-t480s-work/system.nix

@@ -5,7 +5,6 @@
 
 let
   keys = import ../../../variables/keys.nix;
-  mfcl3770cdw = pkgs.callPackage ../../../pkgs/mfcl3770cdw.nix {};
 in {
 
   # TASK: new device

nix/os/profiles/common/configuration.nix

@@ -1,6 +1,10 @@
 { ... }:
 
 {
+  nixpkgs.overlays = [
+     (import ../../../overlay.nix)
+  ];
+
   imports = [ 
     ./boot.nix
     ./pkg.nix

nix/overlay.nix

@@ -0,0 +1,5 @@
+self: super: {
+  duplicay = super.callPackage ./pkgs/duplicacy {};
+  mfcl3770cdw = super.callPackage ./pkgs/mfcl3770cdw.nix {};
+  just = super.callPackage ./pkgs/just.nix {};
+}

shell.nix

@@ -2,7 +2,7 @@
 
 let
   channels-nixos-stable-path = (builtins.fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/7e88992a8c7b2de0bcb89182d8686b27bd93e46a.tar.gz);
-  channels-nixos-stable = import channels-nixos-stable-path {};
+  channels-nixos-stable = import channels-nixos-stable-path { overlays = [ (import ./nix/overlay.nix) ]; };
 
 in
 with channels-nixos-stable;
@@ -10,7 +10,7 @@ stdenv.mkDerivation {
   name = "infra-env";
   buildInputs = [
     (with import (channels-nixos-stable-path+"/nixos") { configuration = {}; }; with config.system.build; [ nixos-generate-config nixos-install nixos-enter manual.manpages ])
-    (pkgs.callPackage ./nix/pkgs/just.nix {})
+    just
     git-crypt
     vcsh
     gnupg