`taskgraph load-task` should cache downloaded artifact images

[matt-boris]

Currently, if you run, say taskgraph load-task Ww-q-IvVTfed8xepUhNWaw twice, you'll see:

2025-09-26 15:54:49,487 - INFO - Downloading from https://community-tc.services.mozilla.com/api/queue/v1/task/MUvi2npBRue4VmksTmsdEw/artifacts/public/image.tar.zst
Loaded image: ci:latest

each time in the logs. It would be useful to cache this downloaded image, as some of these images are large and this takes up a good portion of the time when running load-task.

Subsequent runs would be much faster and help the local debugging/development process.