Add tests for secrets in BuildxCLI to show that they are optional #2648

Job	Run time
build-linux-test-other	31s
build-windows	1m 6s
build-linux-lint	16s
build-linux-test-component (buildx)	2m 20s
build-linux-test-component (test_compose.py)	6m 22s
Render docs	1m 13s
build-linux-test-component (test_config.py)	36s
build-linux-test-component (test_container.py)	7m 35s
build-linux-test-component (test_context.py)	27s
build-linux-test-component (test_image.py)	1m 3s
build-linux-test-component (test_manifest.py)	28s
build-linux-test-component (test_network.py)	1m 9s
build-linux-test-component (test_node.py)	1m 2s
build-linux-test-component (test_plugin.py)	37s
build-linux-test-component (test_pod.py)	54s
build-linux-test-component (test_service.py)	2m 35s
build-linux-test-component (test_stack.py)	57s
build-linux-test-component (test_swarm.py)	41s
build-linux-test-component (test_system.py)	1m 10s
build-linux-test-component (test_task.py)	34s
build-linux-test-component (test_volume.py)	40s
	32m 16s

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