nix config to save some space

nixos/roles/nix.nix

@@ -1,6 +1,7 @@
 {
   nix = {
     settings = {
+      # trusted-public-keys belonging to specific persons are set in rolse/admins.nix
       trusted-public-keys = [];
       experimental-features = [
         "flakes"
@@ -11,6 +12,7 @@
       builders-use-substitutes = true;
     };
     gc = {
+      automatic = true;
       persistent = false;
       dates = "weekly";
       options = "-d";