bug 1597598 - [scriptworker-scripts] use taskgraph

[escapewindow]

We may want to first port the existing logic to taskgraph, and then convert to cloudops downloading taskcluster artifacts rather than relying on dockerhub.

• .taskcluster.yml change to be taskgraph-ified
• decision task
• docker image task(s) - python37, python38 we can likely use directly, but we could add pushapk/pushsnap deps, f/e.
• cached tasks to avoid rebuilding/retesting all scripts if only one script changed
• relpro to promote a docker image to cloudops?
• optional leaf tasks that don't block promotion
• make sure existing checks still work. coverage, ben's standards check, tox -e py3{7,8}, tox -e check
• continue using secrets to publish docker images to dockerhub (relpro?) until we switch away. but also publish as a taskcluster artifact