Add `GANTRY_UPDATE_NUM_WORKERS` and other fixes #43

Job	Run time
shellcheck	9s
Run tests (gantry_cleanup_images_spec.sh)	2m 9s
Run tests (gantry_common_options_spec.sh)	1m 31s
Run tests (gantry_filters_spec.sh)	1m 18s
Run tests (gantry_login_spec.sh)	2m 6s
Run tests (gantry_manifest_spec.sh)	1m 47s
Run tests (gantry_notify_spec.sh)	1m 41s
Run tests (gantry_parallel_spec.sh)	1m 57s
Run tests (gantry_rollback_spec.sh)	1m 8s
Run tests (gantry_service_multiple_spec.sh)	1m 36s
Run tests (gantry_service_no_running_tasks_spec.sh)	1m 42s
Run tests (gantry_service_single_spec.sh)	1m 25s
Run tests (gantry_update_options_spec.sh)	1m 36s
	20m 5s

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