fix: nixPath points nixos-config at master.nix so auto-upgrade works

automatic-upgrades.nix

@@ -5,6 +5,14 @@
   nix.gc.automatic     = true;
   nix.gc.dates         = "03:00";           # run GC at 03:00 daily
 
+  # This host has no configuration.nix: point <nixos-config> at master.nix,
+  # otherwise nixos-upgrade.service (and plain nixos-rebuild) cannot find the config.
+  nix.nixPath = [
+    "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos"
+    "nixos-config=/etc/nixos/master.nix"
+    "/nix/var/nix/profiles/per-user/root/channels"
+  ];
+
   # Enable automatic system upgrades
   system.autoUpgrade = {
     enable             = true;               # turn on unattended upgrades
@@ -14,4 +22,3 @@
   };
 
 }
-