cli-plugins: fix when plugin does not use PersistentPreRun* hooks

This regressed in 3af168c7df ("Ensure plugins can use `PersistentPreRunE`
again.") but this wasn't noticed because the helloworld test plugin has it's
own `PersistentPreRunE` which has the effect of deferring the resolution of the
global variable. In the case where the hook isn't used the variable is resolved
during `newPluginCommand` which is before the global variable was set.

Initialize the plugin command with a stub function wrapping the call to the
(global) hook, this defers resolving the variable until after it has been set,
otherwise the initial value (`nil`) is used in the struct.

Signed-off-by: Ian Campbell <ijc@docker.com>

cli-plugins/plugin/plugin.go

@@ -114,11 +114,14 @@ func newPluginCommand(dockerCli *command.DockerCli, plugin *cobra.Command, meta
 	fullname := manager.NamePrefix + name
 
 	cmd := &cobra.Command{
-		Use:                   fmt.Sprintf("docker [OPTIONS] %s [ARG...]", name),
-		Short:                 fullname + " is a Docker CLI plugin",
-		SilenceUsage:          true,
-		SilenceErrors:         true,
-		PersistentPreRunE:     PersistentPreRunE,
+		Use:           fmt.Sprintf("docker [OPTIONS] %s [ARG...]", name),
+		Short:         fullname + " is a Docker CLI plugin",
+		SilenceUsage:  true,
+		SilenceErrors: true,
+		PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
+			// We can't use this as the hook directly since it is initialised later (in runPlugin)
+			return PersistentPreRunE(cmd, args)
+		},
 		TraverseChildren:      true,
 		DisableFlagsInUseLine: true,
 	}