chore: refactor flake layout

Signed-off-by: Lander Van den Bulcke <landervandenbulcke@gmail.com>

flake.lock

@@ -414,6 +414,22 @@
         "type": "github"
       }
     },
+    "nixpkgs-unstable": {
+      "locked": {
+        "lastModified": 1757745802,
+        "narHash": "sha256-hLEO2TPj55KcUFUU1vgtHE9UEIOjRcH/4QbmfHNF820=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "c23193b943c6c689d70ee98ce3128239ed9e32d1",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nixpkgs_2": {
       "locked": {
         "lastModified": 1757810152,
@@ -587,6 +603,7 @@
         "home-manager": "home-manager",
         "nixos-mailserver": "nixos-mailserver",
         "nixpkgs": "nixpkgs_2",
+        "nixpkgs-unstable": "nixpkgs-unstable",
         "nixvim": "nixvim",
         "sops-nix": "sops-nix",
         "tidalcycles": "tidalcycles"