ci(packaging): let's test the packaging pipeline on PRs too (#11612)

(cherry picked from commit cffc2cd)

# Conflicts:
#	.buildkite/pipeline.yml

Author: v1v

.buildkite/pipeline.yml

@@ -346,3 +346,28 @@ steps:
         - .buildkite/serverless.beats.tests.yml
         - .buildkite/scripts/steps/beats_tests.sh
         - .buildkite/hooks/pre-command
+<<<<<<< HEAD
+=======
+
+  # NOTE: This should help detecting issues earlier in the development cycle
+  # See https://github.com/elastic/elastic-agent/issues/11604
+  - label: "Trigger Elastic Agent Package"
+    if: build.pull_request.id != null
+    commands:
+      - .buildkite/scripts/steps/trigger-elastic-agent-package.sh
+      - .buildkite/scripts/steps/trigger-elastic-agent-package.sh | buildkite-agent pipeline upload
+    if_changed:
+      include:
+        - .buildkite/pipeline.elastic-agent-package.yml
+        - .buildkite/scripts/steps/package.sh
+        - .buildkite/scripts/steps/trigger-elastic-agent-package.sh
+
+  # wait for CI to be done
+  - wait: ~
+
+  - label: "Publish to serverless"
+    branches: main
+    trigger: "agentless-serverless-release"
+    build:
+      commit: "${BUILDKITE_COMMIT}"
+>>>>>>> cffc2cdff (ci(packaging): let's test the packaging pipeline on PRs too (#11612))

.buildkite/scripts/steps/trigger-elastic-agent-package.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+#
+# Create a dynamic buildkite step for running the elastic-agent-package pipeline.
+#
+# Required environment variables:
+#  - BUILDKITE_PULL_REQUEST
+#  - BUILDKITE_COMMIT
+#  - BUILDKITE_BRANCH
+#  - BUILDKITE_PULL_REQUEST_BASE_BRANCH
+#
+
+if [ ! -f .package-version ]; then
+  echo ".package-version file not found!"
+  exit 1
+fi
+
+# No need for the snapshot but the three digits version is required
+BEAT_VERSION=$(jq -r .version .core_version)
+MANIFEST_URL=$(jq -r .manifest_url .package-version)
+
+cat << EOF
+  - label: ":pipeline: Run elastic-agent-package"
+    trigger: "elastic-agent-package"
+    build:
+      message: "#${BUILDKITE_PULL_REQUEST} - Verify elastic-agent-package works"
+      commit: "${BUILDKITE_COMMIT}"
+      branch: "${BUILDKITE_BRANCH}"
+      env:
+        DRA_VERSION: "${BEAT_VERSION}"
+        DRA_WORKFLOW: "snapshot"
+        DRA_BRANCH: "${BUILDKITE_PULL_REQUEST_BASE_BRANCH}"
+        DRA_DRY_RUN: "--dry-run"
+        MANIFEST_URL: "${MANIFEST_URL}"
+        ELASTIC_SLACK_NOTIFICATIONS_ENABLED: "false"
+EOF