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>

e2e/cli-plugins/plugins/nopersistentprerun/main.go

@@ -0,0 +1,36 @@
+package main
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/docker/cli/cli-plugins/manager"
+	"github.com/docker/cli/cli-plugins/plugin"
+	"github.com/docker/cli/cli/command"
+	"github.com/spf13/cobra"
+)
+
+func main() {
+	plugin.Run(func(dockerCli command.Cli) *cobra.Command {
+		cmd := &cobra.Command{
+			Use:   "nopersistentprerun",
+			Short: "Testing without PersistentPreRun hooks",
+			//PersistentPreRunE: Not specified, we need to test that it works in the absence of an explicit call
+			RunE: func(cmd *cobra.Command, args []string) error {
+				cli := dockerCli.Client()
+				ping, err := cli.Ping(context.Background())
+				if err != nil {
+					return err
+				}
+				fmt.Println(ping.APIVersion)
+				return nil
+			},
+		}
+		return cmd
+	},
+		manager.Metadata{
+			SchemaVersion: "0.1.0",
+			Vendor:        "Docker Inc.",
+			Version:       "testing",
+		})
+}