cleanup flake, fix nixpkgs config, and add cosmic flake

nixos/configuration.nix

@@ -3,6 +3,7 @@
 # and in the NixOS manual (accessible by running ‘nixos-help’).
 {
   pkgs,
+  inputs,
   config,
   hostname,
   ...
@@ -18,9 +19,28 @@
         "root"
         "@wheel"
       ];
+      substituters = [
+        "https://devenv.cachix.org"
+        "https://cosmic.cachix.org"
+      ];
+      trusted-public-keys = [
+        "devenv.cachix.org-1:w1cLUi8dv3hnoSPGAuibQv+f9TZLr6cv/Hm9XgU50cw="
+        "cosmic.cachix.org-1:Dya9IyXD4xdBehWjrkPv6rtxpmMdRel02smYzA85dPE="
+      ];
     };
   };
 
+  nixpkgs = {
+    config = {
+      allowUnfree = true;
+    };
+    overlays = [
+      inputs.catppuccin-vsc.overlays.default
+      inputs.nur.overlays.default
+      inputs.gauntlet.overlays.default
+    ];
+  };
+
   # install the most recent stable linux kernel
   /*
     TODO: change this back to 'pkgs.linuxPackages_latest' once