CI: install_script_testing: provide the variables used in the CI script

Instead of providing a variable that will be forwarded through an argument to
another variable

.gitlab/install_script_testing.yml

@@ -7,10 +7,10 @@ test_install_script:
     - source /root/.bashrc
     - set +x
     - export GITLAB_TOKEN=$(aws ssm get-parameter --region us-east-1 --name ci.datadog-agent.gitlab_pipelines_scheduler_token --with-decryption --query "Parameter.Value" --out text)
-    - export APT_URL=$DEB_TESTING_S3_BUCKET
-    - export YUM_URL=$RPM_TESTING_S3_BUCKET
+    - export TESTING_APT_URL=$DEB_TESTING_S3_BUCKET
+    - export TESTING_YUM_URL=$RPM_TESTING_S3_BUCKET
     - export TEST_PIPELINE_ID=$CI_PIPELINE_ID
-    - inv pipeline.trigger-child-pipeline --project-name "DataDog/agent-linux-install-script" --git-ref "main" --variables "APT_URL,YUM_URL,TEST_PIPELINE_ID"
+    - inv pipeline.trigger-child-pipeline --project-name "DataDog/agent-linux-install-script" --git-ref "main" --variables "TESTING_APT_URL,TESTING_YUM_URL,TEST_PIPELINE_ID"
   needs: ["deploy_deb_testing-a6_x64", "deploy_rpm_testing-a6_x64", "deploy_deb_testing-a7_x64", "deploy_rpm_testing-a7_x64"]
   rules:
     !reference [.on_deploy]