should list real pod nodes when all drivers are kubernetes

[morlay]

refactor to remove the strange len(ngi.drivers) == 1, and to check kubernetes driver first

When create builder with different drivers (not matter contains kubernetes driver or not), should list node with endpoint

$ docker buildx create --use --name=buildkit --platform=linux/amd64 --node=local
$ docker buildx create --append --name=buildkit  --platform=linux/arm64 --node=buildkit-arm64 --driver=kubernetes

$ docker buildx inspect buildkit                                
Name:   buildkit
Driver: kubernetes

Nodes:
Name:      buildkit-amd64
Endpoint:  unix:///var/run/docker.sock
Status:    running
Platforms: linux/amd64*, linux/386

Name:      buildkit-arm64
Endpoint:  kubernetes://buildkit?deployment=buildkit-arm64
Status:    running
Platforms: linux/arm64*, linux/amd64, linux/386

When create builder with kubernetes drivers, should list all pods as nodes

$ docker buildx create --use --name=buildkit --platform=linux/amd64 --node=buildkit-amd64 --driver=kubernetes
$ docker buildx create --append --name=buildkit --platform=linux/arm64 --node=buildkit-amd64 --driver=kubernetes

$ docker buildx inspect buildkit
Name:   buildkit
Driver: kubernetes

Nodes:
Name:      buildkit-amd64-85fdfc4898-knqjc
Endpoint:  
Status:    running
Platforms: linux/amd64*, linux/arm64

Name:      buildkit-amd64-85fdfc4898-x9k24
Endpoint:  
Status:    running
Platforms: linux/amd64*, linux/arm64

Name:      buildkit-arm64-bd86845f4-fhzhf
Endpoint:  
Status:    running
Platforms: linux/arm64*, linux/amd64, linux/386

Name:      buildkit-arm64-bd86845f4-vt8qp
Endpoint:  
Status:    running
Platforms: linux/arm64*, linux/amd64, linux/386

Signed-off-by: Wang morlay.null@gmail.com

cc @tonistiigi

[tiborvass]

looks good from a code perspective but didn't test it.

[AkihiroSuda]

Isn't this too much verbose? Can we make this optional?

[morlay]

Isn't this too much verbose? Can we make this optional?

list pod when --debug? but this change the behavhior, not sure does it make sense.
pod is the real node.