CI: make sure to test with the nodejs version we are using in GH

Author: lazka

.github/workflows/Test.yml

@@ -19,6 +19,9 @@ jobs:
       uses: actions/checkout@v6
       with:
         persist-credentials: false
+    - uses: actions/setup-node@v6
+      with:
+        node-version: 20
     - run: |
         npm ci
         npm run test