Fix version of GH checkout

Author: SebKay

.github/workflows/test-webpack.yml

@@ -7,7 +7,7 @@ jobs:
     name: Test production build
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@2.3.4
+    - uses: actions/checkout@v2.3.4
 
     - name: Install Yarn dependencies
       run: yarn install --prod --non-interactive