chore(vscode): select nix env

.vscode/settings.json

@@ -1,5 +1,6 @@
 {
   "editor.formatOnSave": true,
   "rust-analyzer.checkOnSave.command": "clippy",
-  "nixEnvSelector.suggestion": false
-}
+  "nixEnvSelector.suggestion": false,
+  "nixEnvSelector.nixFile": "${workspaceFolder}/default.nix"
+}