consume v2 action during build (#131)

.github/workflows/test.yml

@@ -11,7 +11,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v1 # todo: switch to v2
+      - uses: actions/checkout@v2
       - run: npm ci
       - run: npm run build
       - run: npm run format-check