Add serverless FTR tests to staging quality gate (elastic#167294)

## Summary

This PR adds the serverless FTR tests that we already have in the [QA
quality
gate](https://github.com/elastic/kibana/blob/main/.buildkite/pipelines/quality-gates/pipeline.tests-qa.yaml#L18-L24)
to the staging quality gate.

### Details

We intentionally decided run the same set of FTR tests again in staging
for starters. We're accepting the over-testing here until we have enough
confidence and experience with our serverless product stability to
decide which set of tests to run in which environment.

This PR also explicitly sets the `EC_ENV` and `EC_REGION` environment
variables for QA and Staging. It worked fine for QA env so far without
setting the environment variable because it fell back on the QAF default
values. Setting these values explicitly, makes it more robust.

.buildkite/pipelines/quality-gates/pipeline.tests-qa.yaml

@@ -21,6 +21,8 @@ steps:
     build:
       env:
         ENVIRONMENT: ${ENVIRONMENT}
+        EC_ENV: qa
+        EC_REGION: aws-eu-west-1
       message: "${BUILDKITE_MESSAGE} (triggered by pipeline.tests-qa.yaml)"
 
   - group: ":female-detective: Security Solution Tests"

.buildkite/pipelines/quality-gates/pipeline.tests-staging.yaml

@@ -21,6 +21,16 @@ steps:
         NAME_PREFIX: ci_test_kibana-promotion_
       message: "${BUILDKITE_MESSAGE} (triggered by pipeline.tests-staging.yaml)"
 
+  - label: ":pipeline::kibana::seedling: Trigger Kibana Serverless Tests for ${ENVIRONMENT}"
+    trigger: appex-qa-serverless-kibana-ftr-tests # https://buildkite.com/elastic/appex-qa-serverless-kibana-ftr-tests
+    soft_fail: true # Remove this before release or when tests stabilize
+    build:
+      env:
+        ENVIRONMENT: ${ENVIRONMENT}
+        EC_ENV: staging
+        EC_REGION: aws-us-east-1
+      message: "${BUILDKITE_MESSAGE} (triggered by pipeline.tests-staging.yaml)"
+
   - wait: ~
 
   - label: ":judge::seedling: Trigger Manual Tests Phase"