Nix: write system nixpkgs into registry

roles/common/programs/nix.nix

@@ -1,4 +1,4 @@
-{ config, ... }:
+{ config, systemPkgs, ... }:
 
 {
 	nix = {
@@ -10,6 +10,9 @@ experimental-features = nix-command flakes
 				from = { id = "agenix"; type = "indirect"; };
 				to = { owner = "ryantm"; repo = "agenix"; type = "github"; };
 			};
+			# Write the system's nixpkgs into the registry to avoid mixing nixpkgs versions
+			# https://dataswamp.org/~solene/2022-07-20-nixos-flakes-command-sync-with-system.html
+			system.flake = systemPkgs;
 		};
 	};
 }