fix: use actions/checkout@v2 in test action'

Author: gr2m

lib/create-test-action.js

@@ -22,7 +22,7 @@ jobs:
         node_version: ['8', '10', '12']
 
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       - name: Use Node.js \${{ matrix.node_version }}
         uses: actions/setup-node@v1
         with: