Add self to nix registry

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

hosts/common/configs/nix/default.nix

@@ -1,3 +1,5 @@
+{ inputs, ... }:
+
 {
   nix = {
     settings = {
@@ -5,6 +7,7 @@
       experimental-features = [ "nix-command" "flakes" ];
     };
 
+    registry.self.flake = inputs.self;
     gc.automatic = true;
     optimise.automatic = true;
   };