buildx use default doesn't switch to local

[tiborvass]

When a remote node is added, one expects buildx use default to switch back to local, but buildx ls still shows the remote builder as current.

[tonistiigi]

currently when switching to a docker context with buildx use we will show https://github.com/tonistiigi/buildx/blob/master/commands/use.go#L42 . I wonder if we should show same in this case and leave current behavior of buildx use default only to the case where currentcontext==default