refactor: get rid of global nix flake registry

NixOS/nix#8953.

I don't use it, and it causes unnecessary downloads.

Author: ErrorNoInternet

droid/common.nix

@@ -32,6 +32,7 @@
 
     extraOptions = ''
       experimental-features = nix-command flakes
+      flake-registry = ""
       log-lines = 500
       show-trace = true
     '';

shared/default.nix

@@ -28,6 +28,7 @@ in {
     settings = {
       auto-optimise-store = true;
       experimental-features = ["nix-command" "flakes"];
+      flake-registry = "";
       log-lines = 500;
       show-trace = true;
       trusted-users = ["root" "@wheel"];