Bug 1469610: [taskgraph] Use `action.taskGroupId` to set `ACTION_TASK_GROUP_ID`; r=dustin

This makes it consistent with everywhere else in `.taskcluster.yml` where we
refer to the action task group.

Differential Revision: https://phabricator.services.mozilla.com/D1707

--HG--
extra : moz-landing-system : lando

.taskcluster.yml

@@ -119,7 +119,7 @@ tasks:
                 TASKCLUSTER_CACHES: /builds/worker/checkouts
               - $if: 'tasks_for == "action"'
                 then:
-                  ACTION_TASK_GROUP_ID: '${taskGroupId}'     # taskGroupId of the target task
+                  ACTION_TASK_GROUP_ID: '${action.taskGroupId}'     # taskGroupId of the target task
                   ACTION_TASK_ID: {$json: {$eval: 'taskId'}} # taskId of the target task (JSON-encoded)
                   ACTION_INPUT: {$json: {$eval: 'input'}}
                   ACTION_CALLBACK: '${action.cb_name}'