Only run linting on node 14

This works around an issue with node/no-missing-import that doesn't
properly see that `node:path` is a valid module on Node 12.x.

They are using `is-core-module` to figure out if it is a module or not,
and it seems that `is-core-module` doesn't differentiate between core
modules that are available for CJS (the `node:` prefixed paths are only
available in 14.18+ and 16+ in CJS files) and those that are available
to ESM.

This is really just a bandaid, the right fix would be to send a PR to
is-core-module to allow it to check CJS vs MJS modules (since they are
actually different).

See mysticatea/eslint-plugin-node#299

.github/workflows/ci.yml

@@ -7,6 +7,17 @@ on:
   pull_request:
 
 jobs:
+  lint:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - uses: actions/setup-node@v1
+      with:
+        node-version: 14.x
+    - run: npm ci
+    - run: npm run lint:js
+
   build:
     name: "Node ${{ matrix.node-version }}"