bk(smart exhaustive tests): tune the steps to support GH comments and changesets (#18383)

Author: v1v

.buildkite/smart_exhaustive_tests_pipeline.yml

@@ -1,6 +1,6 @@
 steps:
   - label: "Trigger logstash-exhaustive-tests-pipeline for PRs with qa/acceptance/ changes"
-    if: build.pull_request.id != null
+    if: build.pull_request.id != null && build.env("GITHUB_PR_TRIGGER_COMMENT") != "/run exhaustive tests"
     plugins:
       - monorepo-diff#v1.0.1:
           diff: "git diff --name-only origin/${GITHUB_PR_TARGET_BRANCH}...HEAD"
@@ -16,19 +16,16 @@ steps:
                   commit: "${BUILDKITE_COMMIT}"
                   branch: "${BUILDKITE_BRANCH}"
                   env:
-                    - BUILDKITE_PULL_REQUEST=${BUILDKITE_PULL_REQUEST}
-                    - BUILDKITE_PULL_REQUEST_BASE_BRANCH=${BUILDKITE_PULL_REQUEST_BASE_BRANCH}
-                    - GITHUB_PR_LABELS=${GITHUB_PR_LABELS}
-                    - ELASTIC_SLACK_NOTIFICATIONS_ENABLE=false
+                    BUILDKITE_PULL_REQUEST: "${BUILDKITE_PULL_REQUEST}"
+                    BUILDKITE_PULL_REQUEST_BASE_BRANCH: "${BUILDKITE_PULL_REQUEST_BASE_BRANCH}"
+                    ELASTIC_SLACK_NOTIFICATIONS_ENABLED: false
 
   - label: "Trigger logstash-exhaustive-tests-pipeline for GitHub comments"
-    if: build.env("GITHUB_PR_TRIGGER_COMMENT") != ""
+    if: build.env("GITHUB_PR_TRIGGER_COMMENT") == "/run exhaustive tests"
     trigger: "logstash-exhaustive-tests-pipeline"
     build:
-      commit: "${BUILDKITE_COMMIT}"
-      branch: "${BUILDKITE_BRANCH}"
+      commit: "HEAD"
+      branch: "pull/${BUILDKITE_PULL_REQUEST}/merge"
       env:
-        - BUILDKITE_PULL_REQUEST=${BUILDKITE_PULL_REQUEST}
-        - BUILDKITE_PULL_REQUEST_BASE_BRANCH=${BUILDKITE_PULL_REQUEST_BASE_BRANCH}
-        - GITHUB_PR_LABELS=${GITHUB_PR_LABELS}
-        - ELASTIC_SLACK_NOTIFICATIONS_ENABLE=false
+        BUILDKITE_PULL_REQUEST: "${BUILDKITE_PULL_REQUEST}"
+        ELASTIC_SLACK_NOTIFICATIONS_ENABLED: false