cli: fix builder persistent flag

[crazy-max]

--builder flag should not be persistent for all subcommands.

Before

$ docker buildx version --help

Usage:  docker buildx version

Show buildx version information

Options:
      --builder string   Override the configured builder instance

Now

$ docker buildx version --help

Usage:  docker buildx version

Show buildx version information

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com

[tonistiigi]

It is somewhat nice though that you can do buildx="buildx --builder=foo" and then $buildx build or any other command.

[crazy-max]

will instead hide the builder flags for subcommand that don't need it as discussed.

[crazy-max]

would have preferred to use SetHelpFunc like:

cmd.SetHelpFunc(func(ccmd *cobra.Command, args []string) {
	for _, h := range hidden {
		_ = ccmd.Flags().MarkHidden(h)
	}
	cmd.SetHelpFunc(nil)
	cmd.HelpFunc()(ccmd, args)
})

but it cannot be handled for docs generation unfortunately.

[tonistiigi]

I found an use-case for this in #825

[crazy-max]

rebased since latest changes