Add explicit flatten level

changelogs/unreleased/mc-feature-flatten-ci-scripts.yml

 ---
-title: Add support for YAML acnhors in CI scripts.
+title: Add support for YAML anchors in CI scripts.
 merge_request: 18849
 author:
 type: changed

lib/gitlab/ci/config/entry/commands.rb

@@ -15,7 +15,7 @@ module Gitlab
           end
 
           def value
-            Array(@config).flatten
+            Array(@config).flatten(1)
           end
         end
       end

lib/gitlab/ci/config/entry/script.rb

@@ -15,7 +15,7 @@ module Gitlab
           end
 
           def value
-            config.flatten
+            config.flatten(1)
           end
         end
       end