chore(nixvim): preserve branch-specific setup

flake.lock

@@ -390,12 +390,26 @@
         "type": "github"
       }
     },
+    "nixpkgs_5": {
+      "locked": {
+        "lastModified": 1789844176,
+        "narHash": "sha256-COQmpo4lFIzMgUjr3ntDpjH0NwrCnf7N0uhR+6WPGP8=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "79b35bf0bda5cd110f856aa5b5b2c5ba4460dbf5",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nixvim": {
       "inputs": {
         "flake-parts": "flake-parts_2",
-        "nixpkgs": [
-          "nixpkgs"
-        ],
+        "nixpkgs": "nixpkgs_5",
         "systems": "systems_3"
       },
       "locked": {