Added missing step to GA workflow

.github/workflows/build-elasticsearch-7.9.yml

@@ -6,6 +6,9 @@ jobs:
   docker:
     runs-on: ubuntu-latest
     steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v2
       -
         name: Set up QEMU
         uses: docker/setup-qemu-action@v1