Include unstable channel as overlay

This allows nix cli tools to access unstable from niv’s pinned rev
(instead of having to rely on uncached and unpinned
channel:nixos-unstable). Also packageOverrides might get
deprecated/removed[1] eventually.

[1]: https://github.com/NixOS/nixpkgs/issues/43266

modules/default.nix

@@ -119,18 +119,14 @@ in
       };
       systemd.services.nix-daemon.serviceConfig.CPUSchedulingPolicy = "batch";
 
-      nixpkgs.config = {
-        # Add unstable channel
-        packageOverrides = pkgs: {
+      nixpkgs.overlays = [
+        (import ../pkgs)
+        (final: prev: {
           unstable = import (import ../nix/sources.nix).nixpkgs-unstable {
             config = config.nixpkgs.config;
             overlays = config.nixpkgs.overlays;
           };
-        };
-      };
-
-      nixpkgs.overlays = [
-        (import ../pkgs)
+        })
       ];
 
       # Globally set Let’s Encrypt requirements