Fix failing nix version test scenario by updating pinned version to 2.18 (#1610)

fix: update nix version from 2.10 to 2.18 in test scenario


Agent-Logs-Url: https://github.com/devcontainers/features/sessions/b026e9e9-065b-47f1-aa99-ff8fe8a4ceb0

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: Kaniska244 <[email protected]>
Co-authored-by: Kaniska <[email protected]>

Author: Copilot

test/nix/scenarios.json

@@ -14,7 +14,7 @@
         "features": {
             "nix": {
                 "multiUser": false,
-                "version": "2.10"
+                "version": "2.18"
             }
         }
     },