common: provide a pinned nixpkgs on all infra machines

common/default.nix

@@ -1,10 +1,11 @@
 {
   imports = [
     ./admins.nix
-    ./raito-vm.nix
-    ./raito-proxy-aware-nginx.nix
     ./base-server.nix
-    ./sysadmin
     ./hardening.nix
+    ./nix.nix
+    ./raito-proxy-aware-nginx.nix
+    ./raito-vm.nix
+    ./sysadmin
   ];
 }

common/nix.nix

@@ -0,0 +1,14 @@
+{ lib, pkgs, ... }:
+{
+  nix.extraOptions = ''
+    experimental-features = nix-command flakes
+  '';
+
+  # Provision a useful nixpkgs in NIX_PATH and flake registry on infra
+  # machines.
+  nixpkgs.flake = {
+    source = lib.cleanSource pkgs.path;
+    setNixPath = true;
+    setFlakeRegistry = true;
+  };
+}