Use last good commit when updating serverless submodule

.buildkite/scripts/update-serverless-submodule.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+set -euo pipefail
+
+INTAKE_PIPELINE_SLUG="elasticsearch-intake"
+BUILD_JSON=$(curl -sH "Authorization: Bearer ${BUILDKITE_API_TOKEN}" "https://api.buildkite.com/v2/organizations/elastic/pipelines/${INTAKE_PIPELINE_SLUG}/builds?branch=main&state=passed&per_page=1" | jq '.[0] | {commit: .commit, url: .web_url}')
+LAST_GOOD_COMMIT=$(echo "${BUILD_JSON}" | jq -r '.commit')
+
+cat <<EOF | buildkite-agent pipeline upload
+steps:
+  - trigger: elasticsearch-serverless-validate-submodule
+    label: ":elasticsearch: Update elasticsearch submodule in serverless"
+    build:
+      message: "Elasticsearch submodule update build"
+      env:
+        ELASTICSEARCH_SUBMODULE_COMMIT: "${LAST_GOOD_COMMIT}"
+EOF

.buildkite/update-es-serverless.yml

@@ -1,8 +1,4 @@
 steps:
-  - trigger: elasticsearch-serverless-validate-submodule
-    label: ":elasticsearch: Update elasticsearch submodule in serverless"
-    build:
-      message: "Elasticsearch submodule update build"
-      env:
-        ELASTICSEARCH_SUBMODULE_COMMIT: "${BUILDKITE_COMMIT}"
+  - label: ":git: Trigger submodule update build"
+    command: ".buildkite/scripts/update-serverless-submodule.sh"