Change CI_COMMIT_SHA to CI_MERGE_REQUEST_SOURCE_BRANCH_SHA

Check out of the Merge Request SHA in omnibus-mirror

scripts/trigger-build

@@ -84,13 +84,14 @@ module Trigger
     end
 
     def base_variables
+      # Use CI_MERGE_REQUEST_SOURCE_BRANCH_SHA for omnibus checkouts
       {
         'GITLAB_REF_SLUG' => ENV['CI_COMMIT_TAG'] ? ENV['CI_COMMIT_REF_NAME'] : ENV['CI_COMMIT_REF_SLUG'],
         'TRIGGERED_USER' => ENV['TRIGGERED_USER'] || ENV['GITLAB_USER_NAME'],
         'TRIGGER_SOURCE' => ENV['CI_JOB_URL'],
         'TOP_UPSTREAM_SOURCE_PROJECT' => ENV['CI_PROJECT_PATH'],
         'TOP_UPSTREAM_SOURCE_JOB' => ENV['CI_JOB_URL'],
-        'TOP_UPSTREAM_SOURCE_SHA' => ENV['CI_COMMIT_SHA'],
+        'TOP_UPSTREAM_SOURCE_SHA' => ENV['CI_MERGE_REQUEST_SOURCE_BRANCH_SHA'],
         'TOP_UPSTREAM_SOURCE_REF' => ENV['CI_COMMIT_REF_NAME'],
         'TOP_UPSTREAM_MERGE_REQUEST_PROJECT_ID' => ENV['CI_MERGE_REQUEST_PROJECT_ID'],
         'TOP_UPSTREAM_MERGE_REQUEST_IID' => ENV['CI_MERGE_REQUEST_IID']
@@ -125,8 +126,9 @@ module Trigger
     end
 
     def extra_variables
+      # Use CI_MERGE_REQUEST_SOURCE_BRANCH_SHA for omnibus checkouts
       {
-        'GITLAB_VERSION' => ENV['CI_COMMIT_SHA'],
+        'GITLAB_VERSION' => ENV['CI_MERGE_REQUEST_SOURCE_BRANCH_SHA'],
         'ALTERNATIVE_SOURCES' => 'true',
         'ee' => Trigger.ee? ? 'true' : 'false',
         'QA_BRANCH' => ENV['QA_BRANCH'] || 'master'
@@ -192,7 +194,7 @@ module Trigger
 
       Gitlab.create_commit_comment(
         ENV['CI_PROJECT_PATH'],
-        ENV['CI_COMMIT_SHA'],
+        ENV['CI_MERGE_REQUEST_SOURCE_BRANCH_SHA'],
         "The [`#{ENV['CI_JOB_NAME']}`](#{ENV['CI_JOB_URL']}) job from pipeline #{ENV['CI_PIPELINE_URL']} triggered #{downstream_pipeline.web_url} downstream.")
 
     rescue Gitlab::Error::Error => error