Quality Gates - retry FTR tests (elastic#174342)

## Summary

This PR enables a retry for failed FTR MKI tests when triggered from the
quality gates pipelines.

The retry has recently been introduce to that pipeline for all runs and
has now been modified to be controlled by an environment variable. This
PR adds the corresponding change to the calling pipelines.

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

@@ -11,6 +11,7 @@ steps:
         ENVIRONMENT: ${ENVIRONMENT}
         EC_ENV: qa
         EC_REGION: aws-eu-west-1
+        RETRY_TESTS_ON_FAIL: "true"
       message: "${BUILDKITE_MESSAGE} (triggered by pipeline.tests-qa.yaml)"
 
   # TODO: Uncomment this code when the integration is ready.

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

@@ -19,6 +19,7 @@ steps:
         ENVIRONMENT: ${ENVIRONMENT}
         EC_ENV: staging
         EC_REGION: aws-us-east-1
+        RETRY_TESTS_ON_FAIL: "true"
       message: "${BUILDKITE_MESSAGE} (triggered by pipeline.tests-staging.yaml)"
 
   - wait: ~