[#3399976] Fix OPT_IN_* scope.

Opt in testing of additional branches may not be working. Add missing variables keyword and clear out misleading comment.

.gitlab-ci.yml

@@ -52,12 +52,12 @@ include:
     # SKIP_ESLINT: '1'
     #
 
+variables:
+
     ################
     # Opt in variables
     #
     # These variables allow you to opt in to specific jobs within the pipeline. The current minor is tested by default.
-    #
-    # It is CRITICAL that these variables are indented to the same level as the project above.
     ################
 
     # Set to 1 to opt in testing against the previous major Drupal core version.