Lock checkout version

Author: fwal

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     name: Unit tests
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v2
     - run: npm install
     - run: npm run format-check
     - run: npm test
@@ -25,7 +25,7 @@ jobs:
         os: [ubuntu-latest, macos-latest]
         swift: ["5.2.1", "4.2.4"]
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v2
     - run: npm install
     - run: npm run build && npm run pack-source-map
     - uses: ./