This removes the 'use-artifacts' mechanism in favour of fetches. There are a
few pieces here that need to land atomically:
1. Remove use-artifact related code
2. Call 'fetch-content' from the run-task script
3. Convert existing tasks on top of fetches (jsshell, python unittest)
4. Stop calling 'fetch-content' from toolchain setup tasks (as this now gets handled in run-task)
Depends on D2166.
Differential Revision: https://phabricator.services.mozilla.com/D2167
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extra : moz-landing-system : lando
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