refactor(flake.nix): simplify and consolidate package imports

- Simplified the import statements for `unstablePkgs`, `masterPkgs`, `localPkgs`, `teslamatePkgs`, and `agenixPkgs` by using a helper function `importNixpkgs`.
- Consolidated overlays into `additionalOverlays` and combined them with `extOverlays` in the final import statement.
- Improved readability and maintainability of the flake configuration.

flake.nix

@@ -36,49 +36,38 @@
         let
           config = {
             inherit cudaSupport;
+
             allowUnfree = true;
           };
 
           extOverlays = [
             (nvidia-patch.overlays.default)
           ];
-        in
-        import nixpkgs rec {
-          inherit system config;
 
-          unstablePkgs = import nixos-unstable {
+          importNixpkgs = { flake }:
+            import flake {
               inherit system config;
 
               overlays = extOverlays;
             };
 
-          masterPkgs = import nixos-master {
-            inherit system config;
+          unstablePkgs = importNixpkgs { flake = nixos-unstable; };
+          masterPkgs = importNixpkgs { flake = nixos-master; };
+          localPkgs = importNixpkgs { flake = local-unstable; };
+          teslamatePkgs = importNixpkgs { flake = teslamate-flake; };
+          agenixPkgs = importNixpkgs { flake = agenix-flake; };
 
-            overlays = extOverlays;
-          };
-
-          localPkgs = import local-unstable {
-            inherit system config;
-            overlays = extOverlays;
-          };
-
-          teslamatePkgs = import teslamate-flake {
-            inherit system config;
-            overlays = extOverlays;
-          };
-
-          agenixPkgs = import agenix-flake {
-            inherit system config;
-            overlays = extOverlays;
-          };
-
-          overlays = [
+          additionalOverlays = [
             (final: prev: { inherit unstablePkgs; })
             (final: prev: { inherit localPkgs; })
             (final: prev: { inherit teslamatePkgs; })
             (final: prev: { inherit agenixPkgs; })
-          ] ++ extOverlays;
+            (final: prev: { inherit masterPkgs; })
+          ];
+        in
+        import nixpkgs {
+          inherit system config;
+          overlays = additionalOverlays ++ extOverlays;
         };
 
       wrapUtils = { pkgs }: