ci: fix downstream check (#777)

Author: Neenu1995

.github/workflows/downstream.yaml

@@ -153,5 +153,6 @@ jobs:
       with:
         java-version: ${{matrix.java}}
     - run: java -version
+    - run: sudo apt-get update -y
     - run: sudo apt-get install libxml2-utils
     - run: .kokoro/downstream-client-library-check.sh google-cloud-core-bom ${{matrix.repo}}