rm: reject context builders before loading nodes

[crazy-max]

Current ordering is wrong for offline sockets. It forced driver detection and a daemon ping before rejecting a context-backed builder, which changed the user-facing error from the expected context removal guidance to an unrelated connection error:

$ docker buildx rm dev-mode
failed to remove dev-mode: failed to connect to the docker API at unix:///Users/foo/bar/dev/.docker/run/docker.sock; check if the path is correct and if the daemon is running: dial unix /Users/foo/bar/dev/.docker/run/docker.sock: connect: no such file or directory
ERROR: failed to remove one or more builders

The removal path now checks whether the selected builder came from a Docker context immediately after builder lookup and returns the existing docker context rm guidance without calling LoadNodes.

cc @fiam

[fiam]

Tested and working. LGTM!

[tonistiigi]

This is still needed?

[crazy-max]

No, I don't think so. DockerContext is already set when we synthesize implicit context builders, and docker driver does not allow instances, so the post LoadNodes ContextName() check no longer covers a case that isn't already handled by the earlier one.