Use a worker for S3 imports

Closes #1795 (closed)

• I removed ProcessBuilder.build_iiif, a leftover of #1302 (closed).
• I made ProcessBuilder._build_task assign the ARKINDEX_WORKER_RUN_ID based on Task.worker_run_id, to avoid duplicating code or forgetting to set the environment correctly.
• The system checks wiki page has been updated.

This needs to be tested with arkindex/workers/import!4