Run lint in CI

Author: eregon

.github/workflows/test.yml

@@ -32,3 +32,10 @@ jobs:
       run: bundle --version
     - name: Check that Bundler works
       run: bundle install
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - run: yarn install
+    - run: yarn run lint