Add tests for secrets in BuildxCLI to show that they are optional #2646

Job	Run time
build-windows	1m 18s
build-linux-test-component (buildx)	3m 19s
build-linux-test-other	37s
build-linux-lint	19s
build-linux-test-component (test_compose.py)	6m 22s
build-linux-test-component (test_config.py)	36s
Render docs	1m 12s
build-linux-test-component (test_container.py)	7m 33s
build-linux-test-component (test_context.py)	31s
build-linux-test-component (test_image.py)	1m 34s
build-linux-test-component (test_manifest.py)	35s
build-linux-test-component (test_network.py)	1m 6s
build-linux-test-component (test_node.py)	1m 5s
build-linux-test-component (test_plugin.py)	31s
build-linux-test-component (test_pod.py)	50s
build-linux-test-component (test_service.py)	2m 18s
build-linux-test-component (test_stack.py)	55s
build-linux-test-component (test_swarm.py)	45s
build-linux-test-component (test_system.py)	1m 24s
build-linux-test-component (test_task.py)	1m 7s
build-linux-test-component (test_volume.py)	41s
	34m 38s

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