Ignore syntehtics for mq (#26341)

* feat: ignore syntehtics for mq

* feat: sparse build for merge queue

* feat: update image pointer, add synthetics for mq jobs

* feat: update synthetics preview gh action to be manual only

* chore: revert ci changes

.github/workflows/synthetics.yml

@@ -1,9 +1,5 @@
 name: Preview Synthetics
 on: 
-  pull_request:
-    types: [opened, synchronize]
-    branches:
-      - master
   workflow_dispatch: # allows manual trigger
 
 # Stop the current running job if a new push is made to the PR

repository.datadog.yml

@@ -11,4 +11,5 @@ gitlab_jobs_retry_enable: true
 gitlab_fail_fast: true
 team: webops-platform
 checks_to_ignore:
-  - "Send PR Approval Status"
+  - "Send PR Approval Status"
+  - "Preview Synthetics"