Bug 2009019 - Structure .taskcluster.yml to be a single task

In order to support actions, the `.taskcluster.yml` must be structured
as a single task. This is because when `fxci-config` generates the
hooks, it uses `tcyml.tasks[0]` as the hook task definition.

When we were defining separate tasks for hg and git, then the git
implementation of the Decision task was being discarded.

This refactors the .tc.yml a bit such that there is only a single
top-level task. This is a bit cleaner anyway as it guarantees it will
always be one or the other, with no possibility of accidentally
scheduling two Decision tasks.

Author: ahal