[tests] fix shellcheck. #916

Job	Run time
Test scripts running in parallel	3m 33s
Test container	3m 13s
shellcheck	8s
Test scripts (gantry_cleanup_images_spec.sh)	59s
Test scripts (gantry_common_options_spec.sh)	1m 34s
Test scripts (gantry_filters_spec.sh)	58s
Test scripts (gantry_jobs_spec.sh)	39s
Test scripts (gantry_login_docker_config_spec.sh)	54s
Test scripts (gantry_login_negative_spec.sh)	58s
Test scripts (gantry_login_spec.sh)	1m 2s
Test scripts (gantry_manifest_spec.sh)	51s
Test scripts (gantry_notify_spec.sh)	1m 27s
Test scripts (gantry_parallel_spec.sh)	57s
Test scripts (gantry_rollback_spec.sh)	47s
Test scripts (gantry_service_multiple_spec.sh)	38s
Test scripts (gantry_service_no_running_tasks_spec.sh)	53s
Test scripts (gantry_service_single_spec.sh)	1m 4s
Test scripts (gantry_update_options_spec.sh)	57s
Build and push Docker image	1m 28s
	23m 0s

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