ci: use actions/checkout v2

Author: williamdes

.github/workflows/tests.yml

@@ -11,7 +11,7 @@ jobs:
                 php-version: ["5.5", "5.6", "7.1", "7.2", "7.3", "7.4", "8.0"]
                 os: [ubuntu-latest]
         steps:
-            - uses: actions/checkout@v1
+            - uses: actions/checkout@v2
             - name: Use php ${{ matrix.php-version }}
               uses: shivammathur/setup-php@v2
               with: