ci: disable updating node-version for actions/setup-node (#391)

We want to be using the lowest major version of Node we support in CI,
but still want to get major updates for the `actions/setup-node` action

Author: G-Rath

.github/renovate.json

@@ -11,6 +11,10 @@
     {
       "matchFileNames": [".github/**"],
       "groupName": "workflows"
+    },
+    {
+      "matchDepTypes": ["uses-with"],
+      "enabled": false
     }
   ]
 }