bump actions/checkout from 2.3.4 to 2.3.5 (via #15)

Author: dependabot[bot]

.github/workflows/build.yml

@@ -29,7 +29,7 @@ jobs:
           - "--prefer-lowest"
     steps:
       - name: Checkout
-        uses: actions/checkout@v2.3.4
+        uses: actions/checkout@v2.3.5
 
       - name: Validate composer.json and composer.lock
         run: composer validate