chore: pin GitHub Actions to commit SHAs

Author: nunomaduro

.github/workflows/tests.yml

@@ -15,10 +15,10 @@ jobs:
 
     steps:
     - name: Checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@ee0669bd1cc54295c223e0bb666b733df41de1c5 # v2
 
     - name: Setup PHP
-      uses: shivammathur/setup-php@v2
+      uses: shivammathur/setup-php@accd6127cb78bee3e8082180cb391013d204ef9f # v2
       with:
         php-version: ${{ matrix.php }}
         tools: composer:v2