Allow "docker buildx inspect" to inspect multiple builders at the same time

Same reasons as #380 , but this one is more a "nice to have" and to be more consistent with the other docker commands:

$ docker buildx inspect blissful_swartz charming_chebyshev
"docker buildx inspect" requires at most 1 argument.
See 'docker buildx inspect --help'.

Usage:  docker buildx inspect [NAME]

Inspect current builder instance

$  docker network inspect ffb3f184cd0a 1d41d8457389
[
    {
        "Name": "host",
        "Id": "ffb3f184cd0a2077f75a507320a9613eec135dc6cb234340ea924d576215e96e",
        "Created": "2020-05-04T17:50:40.0997657+02:00",
        "Scope": "local",
        "Driver": "host",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": []
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {
            "a8d13ad9ac75a3343da098003e22052ac6ada63fa03a216e96f549f636b5ab56": {
                "Name": "gabriel_work_env",
                "EndpointID": "fe22d2b01c99eca9992079cfe1bc04774d9f1e29c4c161808273a286cc018ae7",
                "MacAddress": "",
                "IPv4Address": "",
                "IPv6Address": ""
            }
        },
        "Options": {},
        "Labels": {}
    },
    {
        "Name": "none",
        "Id": "1d41d84573897ba7df0dc8fd48bc142d7673362bd8bee870e7bf4f0989b6883d",
        "Created": "2020-05-04T17:50:40.0851246+02:00",
        "Scope": "local",
        "Driver": "null",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": []
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {},
        "Options": {},
        "Labels": {}
    }
]

Start with the docker buildx inspect command and compare its current single-argument behavior with the multi-argument docker network inspect example in the issue. Update the command so multiple builder names can be inspected, and verify that the output covers each requested builder without the current argument error.