switch to fenix to manage rust toolchain in nix

flake.lock

@@ -1,5 +1,26 @@
 {
   "nodes": {
+    "fenix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "rust-analyzer-src": "rust-analyzer-src"
+      },
+      "locked": {
+        "lastModified": 1761806629,
+        "narHash": "sha256-3u8345VliQtbpOtCNYYpDTCsjS8A9osrpU03E8TaIBw=",
+        "owner": "nix-community",
+        "repo": "fenix",
+        "rev": "c7c690951af16e60912678fab6155fb120cc27b0",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "fenix",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1758690382,
@@ -18,8 +39,26 @@
     },
     "root": {
       "inputs": {
+        "fenix": "fenix",
         "nixpkgs": "nixpkgs"
       }
+    },
+    "rust-analyzer-src": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1761739801,
+        "narHash": "sha256-ONUpb+l5oEIb9iOGkmUhze5YjRexZ6sc3mwQyLXlcms=",
+        "owner": "rust-lang",
+        "repo": "rust-analyzer",
+        "rev": "769ebafdc66559d620bdc414743f32bb28180c58",
+        "type": "github"
+      },
+      "original": {
+        "owner": "rust-lang",
+        "ref": "nightly",
+        "repo": "rust-analyzer",
+        "type": "github"
+      }
     }
   },
   "root": "root",