nix: use rnix-lsp in coc-settings.json

.config/nvim/coc-settings.json

@@ -23,6 +23,12 @@
       ],
       "initializationOptions": {},
       "settings": {}
+    },
+    "nix": {
+      "command": "rnix-lsp",
+      "filetypes": [
+        "nix"
+      ]
     }
   }
 }