Add tests for secrets in BuildxCLI to show that they are optional #2637

Job	Run time
build-linux-lint	16s
build-linux-test-component (buildx)	2m 50s
Render docs	1m 9s
build-linux-test-other	30s
build-linux-test-component (test_compose.py)	6m 27s
build-linux-test-component (test_config.py)	38s
build-windows	1m 31s
build-linux-test-component (test_container.py)	7m 23s
build-linux-test-component (test_context.py)	30s
build-linux-test-component (test_image.py)	1m 2s
build-linux-test-component (test_manifest.py)	28s
build-linux-test-component (test_network.py)	1m 11s
build-linux-test-component (test_node.py)	1m 6s
build-linux-test-component (test_plugin.py)	39s
build-linux-test-component (test_pod.py)	47s
build-linux-test-component (test_service.py)	2m 14s
build-linux-test-component (test_stack.py)	56s
build-linux-test-component (test_swarm.py)	46s
build-linux-test-component (test_system.py)	1m 1s
build-linux-test-component (test_task.py)	40s
build-linux-test-component (test_volume.py)	40s
	32m 44s

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