build: fix missing "docker" driver name in build progress

[jedevc]

This was missing, since the driver property can only be fully populated after loading nodes from disk. So we add logic to load the nodes, and check for an error, which ensures that the "docker" driver is always correctly present in the progress description.

I think this was introduced in #1737, when I refactored the progress printing.

Before:

$ buildx build . --builder=desktop-linux
[+] Building 2.2s (10/10) FINISHED                                                                                           :desktop-linux

After:

$ buildx build . --builder=desktop-linux
[+] Building 0.9s (6/6) FINISHED                                                                                       docker:desktop-linux

[crazy-max]

LGTM

Looking at

buildx/controller/build/build.go

Lines 160 to 174 in 69a9c66

	// TODO: this should not be loaded this side of the controller api
	b, err := builder.New(dockerCli,
	builder.WithName(in.Builder),
	builder.WithContextPathHash(contextPathHash),
	)
	if err != nil {
	return nil, nil, err
	}
	if err = updateLastActivity(dockerCli, b.NodeGroup); err != nil {
	return nil, nil, errors.Wrapf(err, "failed to update builder last activity time")
	}
	nodes, err := b.LoadNodes(ctx, false)
	if err != nil {
	return nil, nil, err
	}

it seems we instantiate the builder and load nodes twice now. Might look at this in follow-up.