Bump docker/setup-buildx-action from 3.8.0 to 3.9.0 #110

Job	Run time
shellcheck	11s
Test container	3m 27s
Test scripts (gantry_cleanup_images_spec.sh)	1m 5s
Test scripts (gantry_common_options_spec.sh)	1m 40s
Test scripts running in parallel	3m 28s
Test scripts (gantry_filters_spec.sh)	44s
Test scripts (gantry_jobs_spec.sh)	46s
Test scripts (gantry_login_docker_config_spec.sh)	52s
Test scripts (gantry_login_negative_spec.sh)	1m 2s
Test scripts (gantry_login_spec.sh)	1m 0s
Test scripts (gantry_manifest_spec.sh)	1m 1s
Test scripts (gantry_notify_spec.sh)	1m 13s
Test scripts (gantry_parallel_spec.sh)	1m 0s
Test scripts (gantry_rollback_spec.sh)	45s
Test scripts (gantry_service_multiple_spec.sh)	33s
Test scripts (gantry_service_no_running_tasks_spec.sh)	51s
Test scripts (gantry_service_single_spec.sh)	56s
Test scripts (gantry_update_options_spec.sh)	54s
	21m 28s

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