bump actions/checkout from 2.3.5 to 2.4.0 (via #21)

Author: dependabot[bot]

.github/workflows/build.yml

@@ -29,7 +29,7 @@ jobs:
           - "--prefer-lowest"
     steps:
       - name: Checkout
-        uses: actions/checkout@v2.3.5
+        uses: actions/checkout@v2.4.0
 
       - name: Validate composer.json and composer.lock
         run: composer validate