reference-browser .taskcluster.yml: fix actions

[escapewindow]

Steps to reproduce

Log in to Taskcluster, and attempt to run an action on a taskgroup (e.g., add new tasks)

Expected behavior

The action runs successfully. In the above example, the appropriate tasks would be scheduled.

Actual behavior

Depends on the state of .taskcluster.yml. I've hit an ownTaskId bustage. Once that was fixed, we hit missing index errors. Once that was fixed, we hit a taskgraph: command not found error, and cron tasks were busted.

Backing out the latest attempts to fix in https://github.com/mozilla-mobile/reference-browser/pull/1068.

Device information

n/a