Revert "[Testing] Hardcoding stack build ID"

This reverts commit fb275fb.

Author: ycombinator

.buildkite/bk.integration-fips.pipeline.yml

@@ -25,7 +25,7 @@ steps:
       FIPS: "true"
       EC_ENDPOINT: "https://api.staging.elastic-gov.com"
       ESS_REGION: "us-gov-east-1"
-      STACK_BUILD_ID: "9.2.0-SNAPSHOT"
+      STACK_BUILD_ID: ""
       TF_VAR_deployment_template_id: "aws-general-purpose"
       TF_VAR_integration_server_docker_image: "docker.elastic.co/beats-ci/elastic-agent-cloud-fips:git-${BUILDKITE_COMMIT:0:12}"
       TF_VAR_docker_images_name_suffix: "-fips"