add env

Author: JoeWang1127

.cloudbuild/library_generation/cloudbuild-library-generation-integration-test.yaml

@@ -15,7 +15,7 @@
 timeout: 7200s # 2 hours
 substitutions:
   _TEST_IMAGE_ID: "test-image:latest"
-  VERSION_CODENAME: "bookworm"
+  _VERSION_CODENAME: "bookworm"
 steps:
 - name: gcr.io/cloud-builders/docker
   args: [
@@ -45,7 +45,7 @@ steps:
       # Add the repository to Apt sources:
       echo \
         "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.asc] https://download.docker.com/linux/debian \
-        $(. /etc/os-release && echo "${VERSION_CODENAME}") stable" | \
+        $(. /etc/os-release && echo "${_VERSION_CODENAME}") stable" | \
         sudo tee /etc/apt/sources.list.d/docker.list > /dev/null
       apt-get -y install docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin
       # Install python dependencies