refactor(flake.nix): update nixos-unstable and add nixos-master

- Updated `nixos-unstable.url` to point to the correct branch (`nixpkgs-unstable`)
- Added `nixos-master` input pointing to the `master` branch of nixpkgs
- Imported `nixos-master` in the outputs and created `masterPkgs` variable for potential use

flake.nix

@@ -1,7 +1,8 @@
 {
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/release-24.11";
-    nixos-unstable.url = "github:NixOS/nixpkgs/master";
+    nixos-unstable.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    nixos-master.url = "github:NixOS/nixpkgs/master";
     local-unstable.url = "path:///home/giulio/dev/nixpkgs";
     teslamate-flake.url = "github:teslamate-org/teslamate/v1.32.0";
     agenix-flake.url = "github:ryantm/agenix";
@@ -15,7 +16,17 @@
     };
   };
 
-  outputs = { self, nixpkgs, nixos-unstable, local-unstable, home-manager, teslamate-flake, nvidia-patch, agenix-flake }:
+  outputs =
+    { self
+    , nixpkgs
+    , nixos-unstable
+    , nixos-master
+    , local-unstable
+    , home-manager
+    , teslamate-flake
+    , nvidia-patch
+    , agenix-flake
+    }:
     let
       sysLinuxX64 = "x86_64-linux";
       sysDarwin = "aarch64-darwin";
@@ -41,6 +52,12 @@
             overlays = extOverlays;
           };
 
+          masterPkgs = import nixos-master {
+            inherit system config;
+
+            overlays = extOverlays;
+          };
+
           localPkgs = import local-unstable {
             inherit system config;
             overlays = extOverlays;